In my last KotH run, my only Level 1 trickster teleported to enemy backline every fight. I had 8 other Tricksters all full level 5 with full HP. The level 1 trickster that teleported was positioned at the back of my army - perhaps Impostor is prioritizing the farthest away trickster from the enemies? Not sure...
I have seen this happen other times in KotH also. Impostor seems to work as expected in Brawl and during PvE.