Why do I sometimes not get credit for "Pink'd" when killing bosses?

It could be possible that any followers/allies of yours actually deal the killing blow to the enemy, thereby taking the kill and achievement progress from you. It would seem like an arbitrary reason if not for the achievement that deals with letting your followers get kills on certain bosses, 'I'll Be Your Wingman'.

EDIT: It would appear that followers do not block achievement progress (if they get the kill) when you're in pink armor, however other players will. After the testing we both have done (same results as you had), it seems like this is pretty set in stone. Go solo, get achievement, win at life.