This seems like when you first unlocked the upgrade the lag in the match interfered with the game giving you the upgrade, i don't know if it's possible to get the upgrade back without manually modifying your save data which is already very complicated to do on consoles. Not sure if this can be fixed from happening again because it was due to lag from the host of that lobby, and because there isn't a proper team for fixing bugs and adding content for this game anymore.
I got a question though. Is your party citron past advanced level 9 now, and still doesn't have the overheat upgrade?
Yes my Party Citron is beyond advanced Lvl 9 I got him to the next promotion and haven't touched him since cause of well he needs that upgrade to be good I just wished there was a way to fix it on console granted I know stupid old game but still, I would think if I level him up more I'd get it again but I already know that isn't gonna happen
