Can't heavy stun Xesht

I had a bugged T4 breachstone encounter.
First, I think it should have kicked me out due to timer but it didn't.
Then, Xesht had no down time between his attacks, and they started with disappearing and hunting for me underground. I was unable to heavy stun him, yellow dot didn't appear. Once the bar was full he was normally stunned, but that didn't work out for my build.