This is indeed very common in the S2 and S2a. It is often blamed on the foam above the focusing screen deteriorating, and allowing a pair of leaf springs below the screen to lift the screen up off of its rails. However, I've encountered cameras that retained the problem after that possibility had been eliminated.
I believe a more likely cause is the fact that the mirror is attached to its metal base with foam tape, which also deteriorates with age - in this case allowing the mirror to settle downwards from its intended position. This creates a magnified error, because the mirror is now more distant both from the lens in front and from the screen above, so that the error is greater than the amount of displacement of the mirror.
I don't have enough experience with these to prove a positive cause and cure, but I do have enough experience to think that the mirror attachment is the main issue.