Viper Napuatzi Boss Fight Bug

I fought this boss in Utzaal yesterday using a warrior. When my basic mace attack inflicted a heavy stun due to my crushing impacts ascendancy, the boss was knocked backward through the circle of legionaires and became unreachable for a time. Eventually during one of its flying phases it suddenly popped back through and I was able to finish the fight, but it seems like that shouldn't be able to happen.