Copper Citadel 2nd Phase start is considered heavy stunned???

When phase 2 starts at Copper Citadel, you start off on the ground. Apparently this is considered being heavy stunned. I ran a citadel with a delirious mirror in it. Got to the boss, phased him to phase 2 with the deli still on. When I loaded onto the 2nd arena, I was immediately attacked by all the deli monsters WHILE I WAS ON THE FLOOR WAITING TO GET UP FROM PHASING THE BOSS and killed. It was a 6 mod map so I lost the citadel...

How is phasing the boss not putting the player in an invincible mode while the character gets up? How was this missed??