Designer here.

This is not a bug. Looking at the history, I can see that no dragons received enough damage to be removed (the only damage dealt this round was the Fire dragon dealing 4 to two dragons with 5 endurance, and the air dragon dealing 5 to an earth dragon with 6 endurance). In such a case, both players lose their front dragon, so as to make sure the game doesn't get into a stalemate, which is what happened here.