When an upgrade that changes the appearance of the rat is applied the game doesn't render it properly. The mod pack is Direwolf 1.20 and the info mod that the game has actually renders it properly

Don't know how but its magically fixed

I still have this issue on 1.20.1 (Forge ver. 47.3.5, Rats ver. 1.20.1.8.1.2). If I apply an upgrade to a rat, its appearance doesn't change. Then randomly one rat will change appearance, but the rest that have the same upgrade still don't change their appearance. Happens with the rainbow dye thing too.