Farming 30 reputations with Milla isn't necessary. That's what the limit is for. If you've completed all the quests related to her, then you've probably seen all of her content.
It’s most likely a bug. I’ll remove the reputation requirement from Milla’s quest since it’s confusing — sorry about that. Overall, if your gallery is fully unlocked, then you’ve seen everything. The patch is just around the corner, buddy, so fingers crossed!
