One possibility is that 255 is a maximum. If so, there is only one 2-digit space allocated in your savegame file for that number, and what may be happening is this. If you set a level at 255 and a promotion tries to increase it, then it adds to the FF of your points. The problem is that a number >255 needs 3 hexadecimal digits to express it. So the FF is converted to something near zero, and there is nowhere available to put the third digit - or possibly if there is a slot available for the 3rd digit, the game engine is not programmed to look for it.
A broadly similar thing happened in the early versions of MM6; if any ability goes over about 60 (may be 65) - any effect dependent on that ability drops like a stone. I don't know if they corrected that in the later versions.
I'm not sure how you put it right. Trouble is, I don't know if the effects are wider-spread than what you've observed so far. You may have started a corruption in your savegame file, and if you have, it will get worse as you continue to play and save.