LS4: Instance "The Hero of Istan": Potential Bug? (no spoiler)

I just did that instance and had a strange "problem": When leaving the first part to enter into the second one, a foe from first part did follow me. Strangely, it was not possible to kill him, because he was not really "existing" (no marking of any sort, neither yellow nor red nor else, and no targeting possible). However, he was continuously aiming at me, dealing bad AOE damages for real! He was following everywhere, sticking to me like glue, during the whole fighting in second part and even after that during the blabla! That was pretty annoying... He stopped following not because he was tired of me, but because while running to the 3rd - and last - part, I did achieve to get him stuck on some crates on the path! Crazy... =)

Anyone else had that?