It looks like the light fixture shown in the approximate roof rafter leading lines roof scene center is switched on, or is reflecting another light source (sun, general daylight from another source outside the scene, or a source of other light coming to its presenting center glass-like cover for its reflection or projection of light into the camera lens of choice glass surfaces and on to the camera sensor). All the other in scene subject illuminance looks fairly even from available light, except that center bright slightly off axis whiteish illuminated imperfect circle's ghosted light seemingly bleeding onto its nearby area. So the exposure value of that light as an illuminance source itself and as a part of the scene coming to the lens differs from exposure value of all else presenting in this scene (without any such similar illuminance by projection or of a brighter source reflection). The rest of the scene acts as a kind of darker background to the imperfect center circle bled-cast light of a brighter illuminance, in addition to it being its own larger part of the scene. A light source seems to be reflected as smeared and diffused on to the roof surrounding background and/or on the capture lens glass element(s) surface as well.
All else we see evenly presents similar illuminance values, and embraces the lens attributes achieving its capture for an adequate rendering, whereas the light source of contrast smears its presence into the otherwise geometric roof scene one way or another. Is this then excessive flare due to format size 'lenses', as the poster wants to know?
I have a quantity of 645 Medium format MC Mamiya lenses, and Pentax and other multicoated lenses for full format and APSC format. They are not flare prone in everyday conditions. They become so when a light source of notable reflection or projection is part of a scene with a higher area illuminance value than in the rest of the scene. When showing up as more about a direct light source itself than a scene in itself, flare can appear in a scene challenging a lens to reckon its associated light controlling attributes as acceptable for the photograph's intention (where its presence is not presented out of sorts with the rest of the scene). The challenge of how to deal with such flare happens with focal length lens choice, use, and photographer placement of his system's light capturing tools juxtaposed with finesse to the axis of a scene's in scene light source.
This is not about a format size available system lenses, but often can be about a specific lens engineered design, as is made for its angle of acceptance, contrast handling of a light source appearing in a scene going above the scene illuminance values charted potential for a comfortable result, a lens component internal light controlling parts being off axis to the scene's direct light illuminance source(s), and glass elements surfaces shapes angles inside a very wide angle lens (as differ from those with less curvature of internal elements to achieve more narrow angles of acceptance for other system interchangeable lenses: elements as may be set inside of a lens barrel by its elements or aperture components light handling design).
Off of direct axis to a subject or scene, an unevenness of light coming from a direct in scene high light value source, or being cast onto a scene background, or coming onto a lens element or element group surface within a lens barrel can challenge any lens internal geometry to render an acceptable photograph without flare (as to mastering the issues of any of its abilities employed when using such an optic). This one involves a light source disrupting a simple appreciation of the scene rendered. And, for the 645 format, it is shot by an extreme wide angle lens. A 35mm 645 lens has a bulb-like highly rounded element or elements group near the very front of the lens. It must be accounted and compensated for as to how the camera is positioned regarding its lens axis to the subject angle and the presenting problem light source in the scene, as well as the focal length used for capture. The presenting bleed cast, I believe, is falling on the very front side of the bulb like element or group of the lens, and then falls off for a literal distance from its top side presenting the light source onto the element(s) group, and smears evenness of light capture by the element shape front toward back of the lens employed in reference to the whole scene's otherwise even illumination presentation. The element could be a hybrid element, made of different material than other elements in the barrel, and is presenting to the scene with its own contributing characteristics of handling rendition and reflection off its own internal to the lens extreme shaped bulb-like internal group. This is not about the format size, but the challenges to designing and handling this kind of lens, and its composition framing of scene by a photographer's very careful placement regarding the light source to camera location in the scene capture environment.
The new Pentax 11-18mm f2.8 APSC ultra wide zoom costs twice as much as its earlier brother, the 12-24mm f4 DA. It has greater aberration control abilities than the earlier DA lens, uses a different engineered optical elements design, offers a slightly different focus mechanism, and certainly more costly elements glass or glass substitute composition in its design. It costs much more too for its extra f-stop for light capture than the earlier unit. It seems to have less flare as well when in such direct light challenged scenes. It masters good edge to edge sharpness. This too is not about its format size, but its own design and engineered composition of elements and aperture control. These two lenses also have frontal bulb-like elements. I have the older one, which has similar flare challenges to the 35mm 645 lens.
Because of the number of elements contained in a camera lens barrel, and the shapes of various elements and element groups of various lens glass compound and complex groups making up the focal length: all and any photographic lenses differ from one another regarding handling flare characteristics. They are placed in alignment, are centered, and shaped for their compound positions which make up a focal length and aberration control and light handling abilities for a lens. Within a lens barrel confines we know not all lenses can do their light gathering, spreading, and focusing exactly the same in all available formats lens systems when placed on any camera.
Photographic lenses are made to achieve a complex optical design for a focal length, for handling, scenic light admission, transfer, evenness of light across the frame, to keep all in a scene in acceptable spatial perspective relationships -- with controlled distortion -- and so to rear project a sharpened color imaged area of a lens focus onto a light sensitive rendering flat surface. All achieved presents challenges to overcome not just flare, but of a scene to the spheres of the various glass surfaces in the barrel held in juxtaposition to a scene (which you presented above with varied light sources, and exposure value differences from the rest of the scene. What we see indeed reflects off a definitive glass surface of an inner element or element group of almost a bulb-like shape which ghosts and smears its own capture of the light source reflection or projection coming into and onto the lens element itself: and so presenting in contrast to the even light of the rest of the scene. Likely as you go longer in focal length, say to a 45, 55, 80, 105, 200 mm 645 format lens, with a less extreme bulb-like shape, the phenomena will lessen or disappear. And, if you shift the axis the photographer controls of the light source to the near center of the lens used and its front bulb-like inner element(s) you should get an acceptable variance on the light smear effect (when it strikes more to the center of that element, and less to one of its bulb-like sides). This issue would not be akin to the format but to the lens type itself; it is not then as conjectured to be about the format's interchangeable lens system.
There is no such animal as a totally flare proof consumer lens: depending on the illuminance light source, the exposure value difference between all else in the scene and that direct light source, the inner lens elements shapes, these elements compositions, and designs, the lens elements coating, the aperture design, and the juxtaposition of the presenting scene light source or reflective source to the axis of the lens both to the sensor plane and subject being photographed. The trade offs to achieve an acceptable photograph using any system's available tools and technology are legion.
As to this photograph, the photo tools being used are not directly below the light source, but are off axis from the lens center: the sensor full plane. where all these center at an angle to the light source. Of course the elements in the UWA lens so disposed will render what presents as was set up pointing toward the light source in this scene. You can move the lens and camera around to show more or less reflection off of the element(s) of the front most behind the first element bulb-like element or element group. What is not presented as perhaps essential to your question is any real comparison to other alike types of UWA lenses in other formats photographing the same scene to then make the hypothesis so that what we see is because of the medium format and its available lenses somehow being more flare prone than other formats. No evidence is presented to prove this.
Your hypothesis is that this is because of the lens to format reality. Likely a 20mm or 24mm on a full frame camera can replicate this rendered result, if set up exactly like the camera and with a similar format to format UWA lens, with its alike on scene camera support, with the same basic angle to the source of illumination as was so positioned to this lens (and likely too with such a bulb-like near the lens front element or element group. You are requiring a flare to format conformity by the principle that your question is correctly or authoritatively framed, when it is not likely just an issue about what you have proposed it is about (format and format lenses flare tendency). When we think through the use of tools to get the shot, and all other factors contributing to the render you show, than simply a lens to format flare factor -- we have to form a different and other formats comparison hypothesis as to what causes and how to lessen such flare.
