Companion vanishes during Boss Rush

Every Boss rush I've run, my companion has vanished at some point. It happens after killing a boss and teleporting to the loot room. I turn around and my companion is just not there. I've had it happen as soon as after the 1st boss, but it occurs randomly and usually in later waves. They remain gone for 2-8 bosses, and when they return it can occur during a boss battle or in the loot room. I've successfully hung out with a higher wave, non-aggro'd, Humbaba and had the companion appear, but that has only occurred once. So far I can't find a pattern or any steps to reproduce. It has happened without them being downed, and once where they had not yet taken damage. I'm playing a crit-stab build, but they vanish equally with fast or slow(er) kills.