Posting on behalf on Tyler;
I've never seen this bug in coliseum before. Wave 2 had a Unicorgi in it, my team had a Spectral Drake, and the wave timed out with just the enemy corgi and roller warrior alive, registering me the wave victory. That is fine and normal. But, for wave 3, that very same Unicorgi strolled out on my team and decimated the enemy - no corgi should have been in play on either side. It was as if my spectral drake possessed it after wave 2 had ended and paraded it out!
I didn't get a screenshot as the battle ended too quickly, sorry!