Ether Resupply Public event bug in Nessus Exodus Black area.
When you damage big Servitor until 1st of 3 health bars drop, it becomes invincible and 1 small Servitor is spawned as usual. But after killing the small one, large Servitor is still being invincible and you cannot damage it anymore. All you can do is wait for the public event timer to expire.
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This has been there for so long (and public events having been neglected) - and we know they have been aware of this for years. This will never see a fix.