I have also noticed this in Lornar’s Pass with the “Defeat the Brimstone Imp & Summoned Imps” event that takes place in Highfirn Passage near the POI Rocklair and its scout. All the imps operate normally except for Brimestone himself. More often than not he is invulnerable. I almost managed to kill him once, but he managed to outlast me. Sometimes he is invulnerable for a handful of strikes, but most often is invulnerable during the entire fight.