After you damaged a bit the giant servitor, it spawns a small one and becomes immune. After that small one is killed, normally, the giant servitor loses its immune shield. But sometimes (I assume in cases when players have dealt too much damage) the giant servitor keeps its shield, failing public event. This issue first time was introduced literally 2 years ago and still no fix!
I just did this event solo and barely damaged the prime ether servitor (about 20% damage Id say) and somehow managed to bug its immunity. I think it has a weird interaction with DOT effects because I was playing with Thorn and Necrotic Grip and literally never had it happen before picking up this playstyle.