I have a sneaky suspicion that most lenses are purposely designed (by Pentax and maybe some others) to provide the best bokeh at certain camera to subject distances, for what the lens was designed for like macro or portraiture, and at certain apertures.
If you take a macro lens for instance, the designers will more than likely opt to have the bokeh look good at the closer focus distances and maybe the smaller apertures like f8, f11 etc, and also the optimum sharpness etc.
It is also very important where the background OOF areas are compared to where the subject in focus is and also what the background is made up of. Highlights can play a detrimental role to some lens's bokeh yet not others.
If we take the FA77(a portrait lens), I find that it's best bokeh occurs at the middle camera to subject distances and the larger apertures like f4 and f5.6 etc.
The FA43 can have some beautiful bokeh at certain apertures and camera to subject distances, but at others it can be quite ordinary. The FA31 generally has good bokeh, but can be a little ordinary at some apertures and C to S distances.
I notice that with some lenses and in some shots, OOF areas behind but near the subject can have pleasing bokeh, but further behind can have poor bokeh and visa versa in some instances.
Some of the zooms can have excellent bokeh at some focal length/aperture/C to S distances and be very ordinary at others.
There are not too many lenses that are good at all camera to subject distances and all apertures. Some may, not all will. I also think that colour can play a part in how bokeh is perceived and how the lens resolves it. I find that in some cases, playing with the white balance can affect the bokeh to a small degree too.
DFA100mm f2.8 macro:
The FA*300 f4.5 sometimes has good bokeh and other times not so good depending on C to S distance, Aperture used and background highlights: