I have noticed the same type of bug with Andariel and Beast In the Ice.
When both bosses leave the arena, you then have enemies (or totems) to kill. I am playing an Ice Build Sorc, with a companion. I feel if my companion or if a specific (not sure which) ability kills one of these enemies, the boss will just never come back, and you are just stuck waiting and need to kill yourself to reset the boss. I have had this happen 3-4 times in beast in the ice recently, and this occasionally happens in Andariel too and this is just annoying! (There are some bosses with the same type of mechanic that I have not been able to try on T3)
Youtube video:
Beast in the Ice Video.
Boss attempt at 1:10.