Simulacrum Player Becomes Untargetable

I've been running +4 simulacrums this morning and, to the best I'm able to discern, I become untargetable by monsters after killing certain rares with the Behead support.

I was not observing this behavior prior to using the Behead support gem, but after slotting it in (ice strike) and killing some rares during the first 1-2 waves, I'm consistently becoming untargettable for all remaining waves. I'm not sure if it's necessarily "untargettable" but I cannot be hit by the monsters. They just wait at the edges of each area until I get close enough/attack them, then run at me but never actually hit me - almost as if I have 100% evasion. As far as I know, I can still take damage to something like burning ground. The monsters still push me around like I'm on a freshly zamboni'd skating rink.

Another instance I've observed just once (I believe also affected by Behead) is that burning ground permanently spawns for the remaining 15 waves after killing rares with certain modifier(s).

Unfortunately, I don't know the name of the modifier on the rare that actually causes this to happen but it seems like it's not exactly modifier-specific anyhow.