Rathbreaker boss fight bug

Hi, i am trying to defeat rathbreaker for the start of act 2 quest. he aggro-ed when i went up to hit plataue and when he was nearly half dead he knocked me down the hill, then he unaggroed and even when i leave to town and return, he doesn't aggro. his bossbar appears with full health, i can stand next to him hitting him infinitely he and he infinite heals it off and the boss bar doesn't move more than a smidge then instantly refills. i have a video, not sure how to attach it.

This seems to be a common issue with Rathbreaker since EA release.

It is thought to be most likely to occur when you leave the hill, but it also happened when I last fought him in Cruel without ever leaving the hill. I was right beside him when he de-aggroed, staring down from the hill and constantly regenerating.

I had to use Restart at Checkpoint to restart the fight.