This is exactly what happened to me an hour ago.
Instant dead in Pit 40. “Unknown Killer”
Also a whirlwind barb. Overpowered for this level. I made this level many times using nothing but the mouse without any visible life decreasing.
Even the boss of the level couldn’t one hit me with the barrier off.
If it is a bug, I give up playing. It doesn’t make much sense for me.