If the opponent plays an Overlord as a duration card, it will appear as plain old Overlord on the player's mat, and not the card it was played as.
There's probably the same issue for Band of Misfits too.
This is at least a display problem. Can you say if the Overlord card behaved correctly, that means if the bonus during the next turn was correct?
The effect of the card was correct, it was just a display problem. Still annoying to not be able to easily check what it was played as.