Sea Witch Vulnerability + Heavy Strike seems to ignore Unwavering Stance.

tl;dr I died to hillock in a white 66 map.

Heavy strike, in particular, seems to be capable of stunning you if you're cursed with Vulnerability. Much like how some enemies still cast Projectile Weakness, the Sea Witch version of Vulnerability still seems to increase your chance to be stunned... even if you're immune to stun.

Maybe the knockback is triggering some kind of forced stun that can't be prevented? I couldn't even use flasks, I just stood there stunned 3 hits in a row. No latency issues.