Im not sure if its connected with free action potion/living free action potion or other bugg he exploited.
Posting one screen I have made. Showing his nick bcs he admitted to doing some kind of abuse.

As you can see on this screen shot I have him on target, just used kidney shot and there is big yellow "Immune". You can see also that he has no free action icon buff or other buff/debuff that would grant him stun immunity. Situation like on this screenshot was taking place for at least 10 minutes.
Dont get me wrong I am not creating this thread to make some kind of delation. I am just afraid that if more people will find out that bug or whatever it is it will be exploited by many players - imagine doing pvp than...
I'd erase that player name but as he admitted to abusing something Idont see any reason for doing this.
Hope it will be fixed so it will be no longer possible to do such exploit.