Hmm I had a problem with my K-r where it would randomly underexpose (pretty severely, very dark photos) in bright high shutter speed conditions.
Here is how I described it then:
"When out shooting in the daylight, I would take a photo of a subject (well lit, for example a no parking sign on a brick wall), sometimes the image would show as almost black (very underexposed) on my screen, next shot would show perfect exposure, but the ISO, shutter speed ,f stop, settings, lens and subject were all the
same from one shot to another. AF, center point, jpegs. This happened with a couple of lenses (F28 and DA15) many times during a day of shooting, seemingly randomly, I dont think the AF was choosing a dif subject for focus. You could take 5 shots of the same point and maybe 3 would be black and two good, then you could take another 10 and they would be ok, it popped up randomly."
I couldnt find what caused the issue, neither could Pentax, so they just replaced it. I have a K-5 now
Not sure if this is the same issue or not. The thread for my issue is here, not much response to it and sadly I deleted the sample photos. But they were almost black. The camera would take great shots and then randomly produce a dark shot.
https://www.pentaxforums.com/forums/general-technical-troubleshooting/150836-...tos-black.html