I think there is a bug. One of the event quests is to level up the event rare card to lvl 5. I have her at lvl 17 and it's impossible to level her up for me now.
It's been like that since the beginning, the developers have these events set to repeat without bothering to acknowledge that some of these quests have already been completed, so we have to level up the card in question again. It doesn't make any sense why the game doesn't recognize we've already done that, it should have been coded to recognize that no matter what, but for whatever reason the game doesn't do that.