It's not a bug, it's a last minute feature that wasn't mentionned in patchnote.

Some players raised the issue that with a large harem, you don't get any more white and green affection items. And that might result in problem to fine tune the affection you give to your girls. Hence this system that gives a chance to get rare items even with a very large harem.

Honestly, I think it was a mistake. PvP provide enough white and green affection items. They should take off this feature, imho.