Unable to target or damage Zalmarath

Experiencing a bug with Zalmarath on my new Huntress playthrough.
After the first 2-5% of damage, Zalmarath is unable to be targeted or damaged by any attacks. He is still able to damage players.

Things i've tried so far:
- Tried new instance of the area and ran through again
- Tried all abilities, including area attacks (twisters)
- Tried inviting other players to assist and they could also not target or damage him
- Re-installed the game

None of the above resolves the issue.

Short video showing spears going straight through him and inability to target him which may be of help.

https://youtu.be/mBk-g7Sw2as