Act 4 Prisoner Boss - Ballista bug?

I was just stuck fighting the Act 4 boss for quite a while with the ballista appearing to not work.

No matter how fast I hit the lever, he would tank the spear but heal to full - meaning I could not end the fight in the last phase.

I saw somewhere people mentioning needing to reset the zone and try again, I did this twice. The two times I got a boss arena with the entrance on the south-east, the ballista wasn't working correctly (on many different pulls of the boss). The final time I rolled a boss arena with entry on the south-west side, this time the ballista worked perfectly and boss died easily (definitely could be a coincidence).

Same, i tried resetting the zone few times but it kept heal to full no matter what and i wasted 2 hours of my time on this bs.