So, after moving around a bit, the second mirror boss immediately teleported to the lower-right hand corner of the room and got stuck there. It was pretty hard to damage him during his first phase because of this. When he transitioned to the second phase and went under the mirror, there was nothing I could do. All I could see were his feet moving around and a Guppy's Hairball floating in the corner.

If there weren't enough shards left to damage me, this was effectively a soft lock. Exiting and continuing resulted in the same error.