The Primalist Skill Thorn Burst doesn’t work in certain areas/there are “invisible walls” where the thorns can’t enter. The sourrounded Area blocks all thorns.
The same happens around pretty much every NPC that doesn’t have a health bar. I assume that is intended, to not “attack” the NPCs. But there are other areas with crates/barrels that you can’t destroy with thorn burst because of this.
(I think this is a general thing of specially “tagged zones” for NPCs or something. I’ll save all places I come across, but won’t post them here unless you ask for it, to not flood the thread.)