This happens to me aswell but on PS4. For some reason I'm not allowed to upgrade any star card to Epic (purple) when i reach level 25 (this happens to me with every heroe/villain). I solved this by joining a Heroes vs Villains match, and before the round starts, i upgraded the cards in the pre-round menu, the bug never occurred in the pre-round menu, only in the collection menu. As soon as i upgrade a star card to purple, the bug disappeares and i can upgrade them to purple in the collection menu too. This works on PS4 but maybe it works on PC too, give it a try, i hope this helps
