Same issue is discussed here:
https://forums.ootpdevelopments.com/...d.php?t=360825
It seems that the actual red shirt tag does get removed when they leave NCAA, but the effect still remains - and never goes away. There doesn't seem to be any fix that a player can do. You may ask devs to fix your save file though.
I really hope this issue gets fixed in a future update as it's quite the hassle when it happens to a good prospect. It effectively removes that prospect from the game.