[Known] Minions Trapped on Previous Level

I was working my way through one of the underground levels (the first one right after the glyph quest). I had summoned my zombie to help me fight. I don't recall the time limit on zombies (level 2 or 3 gem at the time), but I had my zombie up for about 20 seconds by the time I went to the sub level with the boss.

I spent about 10 - 15 minutes in the sub level (I took a bit of a break to take care of my kid) and then went back up so I could continue on and found my minion waiting for me. I assume that this is not the intended behavior.

On a side note, it would be nice if minions traveled with you from level to level.