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This happened once to me a few months ago. Unfortunately I have no idea how to reproduce it, but I do remember it was a Hiveguard that suddenly stopped in place when it should have spawned sentinels. It continued to display a gentle swaying animation and idle moaning noises, but never moved or attacked. Out of 336 elimination missions that I played, I only remember it happening that one time. We eventually aborted the mission since there was no way to damage it.

He stuck when i damage 1/3 of his hp, and when i kill another dreadnought he still bugged but spawn sentinels, so i could damage him again but this is no worth