Sounds like on the M8, then, the 50 is focusing nearer than you wish, or farther than you wish. And at small apertures the depth-of-field masks the effect. You might try to determine how far off it really is, and which direction, wide open where DoF is at a minimum and focus is most critical.
There are several ways to do this... approximately by pointing the camera at ~45-degree angle to a wall or floor which has a distinctive focus point and some texture. Shoot close-up at f/1.4. On the photo you can then see about how far off the RF focusing is. More precisely, you can rig up a 1m or 1yd rule and focus (again at a 45 deg angle) on the middle. Then on the picture see which division line is sharpest. There was a post here some months back showing a folded paper construction that achieved the same end but had some dimensional tricks to minimize the size/perspective effect of the varying distance of the near/far parts of the target.
Seems to me having more specific information will help you to determine the nature of the problem.... and in describing the problem to the technicians doing the repair/adjustment (which I'll just guess is going to be needed).
