I ‘ve initiated a fight with the grub in the Valley keep at Eternal battlegrounds just about 20 mins ago and it wouldn’t fight back. I don’t know how to reproduce the bug but just in case it helps I captured in screenshot the exact position I was when I attacked it. I also might have started the fight with an immobilize skill, Shockwave of Elementalist but I ’m not 100% sure. The moment an other guy attacked him, it appeared not attacking back at first, but then it started fighting. Also, I should note that while fighting him I was always in and out of combat, for no apparent reason.
Edit: I ‘ve reached him at about 50% before more people came in, so it isn’t un-bugging even at phase changes, where it is sposed to dig into the ground and appear somewhere else (He didnt spawn grubs either). I assume I could have solo him without ever getting attacked.