It's a 'permanent' adjustment to his ratings for the following season. Of course if he suddenly hits good against lefties again next season it will raise. The more I think about this and reading the post you quoted the more it bothers me. I don't understand why they have to make things so major, just make it a +4 or +5 max increase/decrease. Or give us a slider to control the amount of increase/decrease like NBA2k lets you have sliders for pretty much everything. Maybe there are people who want that large of a decrease but I know many of us don't.
The code is locked but they are always great about patches improving gameplay, this is something we need to raise noise about and maybe it will be tweaked.