So I've gone back a few times just to make sure, and when a lot of people are in the archons Forge doing a bunch of offerings sometimes the enemies do not despawn and become immune when the next one is put in. so after a perfected captain we'd beat it and there would be reaver captains that wouldn't despawn and when we put in the next one We have invincible Reaver captains running around.
The bigger issue seems that because those enemies can't be killed more do not spawn from that door/spawn point. 2 or 3 times since feb 21 for me
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That's normally caused by lag when you get upwards of 6-9 guardians in the Forge. The server host's machine can only handle so much going on at once and when you do things faster than their system can easily process, hiccups start to happen. To avoid it, don't rush to immediately put in another offering before the previous one has finished its cleanup phase.