couldn't it be a pure electronic problem? All M exposures on my roll were 100% ok. I agree this is without use of the stop down lever which acts only in S and P.
I made all A and P exposures identical to my M exposures . So the meter performed ok in M which was the same exposure on the meter in P and A.
I made some shots of the aperture's action in S. See attachements. I could clearly see the aperture closing at max f22 when pointed to the light blue sky in spot metering mode, shutter speed on 2s and 4s . When pointed to a dark subject the aperture widens up to almost maximum f3.5 in this case.
I really appreciate your input!