I think this issue has been reported before but it is still not solved.
When killing an enemy the card remains on the field, not only that it sometimes happens with the hometeam as well.
I can’t say when it happens because it seems to be a random event. It happened to me while playing Dungeon, Raid, Explore, PvP, Petrescue… Well in every part of the game even in the newly added Halls…
I play on Steam/PC