I’ve also encountered this. The worst part of it is that it can be done at any time of day, and on PvE-C you can’t even respond to it as the Thralls - rather stupidly - are classed as buildings and can’t be hurt.
I really wish PvE-C had different rules for Thralls so they respond as they would during raid hours in PvP, and also to make sure they weren’t indestructible during those times. Unless ordered to attack, all they do when it comes to PvP hours is stand there and watch you when it comes to player encounters… Why aren’t you defending me and my territory?