I can see this has been reported back in '19 and '20 but not recently. I’ve never seen this bug before and it’s currently broken the cow lvl I am in. We’re 2 players playing together and the mob is stuck at 1 hp for both of us. We can hit it etc, but can’t kill it. Since it’s in cow lvl, we can’t really “just” do any sort of reset. But because it’s one of the mobs from the event inside the cow lvl, we now can’t complete the event! 
We’ve experienced this SEVERAL times since returning to the season yesterday, but it hasn’t ‘broken’ the game for us until now. So it’s nothing to do with cow lvl, and we have closed the game and reopened several times.
Let me guess, you play a wizard in this season and use Black Hole? It’s a known bug for a long time, but blizzard never fixed it. Mobs get invulnerability in a short time frame, when you are using Black Hole, and then they are unkillable.
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11/4/2023 @ 3:25pm - doing final Bounty: The Cursed Realm, Fractured Fate Battlefields of Eternity, Cursed Chest, unable to kill Scouring Charger (Ancient Beast) to open Cursed Chest, Kill Remaining Enemies. I am playing Wizard on Torment VIII, with Firebird Flame Blades build 2023-11-04T05:00:00Z
Read the post by @Frostbyte, just above yours.
The issue occurs on more than just Black Hole wizard… but they were probably the first one to encounter that issue.
I don’t know if it will ever be fixed…