This is the second time I've seen this happen. When we started to engage boss, only Heston moved. After we killed him, no boss loot drop because Preston was still alive. But we couldn't damage him with skills, pets wouldn't attack him. The only things that hurt him were weapon proc (in this case Glintstone bow bombs) and pet DoT (Slobber puke). We had no mage in party. In my previous encounter with this bug (?), we had a mage who could clock the unkillable Preston. It took a long time to whittle away his health with these.

You can see in this screenshot that Preston had not moved from his spot.

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