iPande TNH unkillable bug

Been trying to solo TNH (The Night Heart) in iPande lately however he continually heals to full hp.

I’ve tried clearing the area of all mobs
Making sure there’s no other mobs that could complete heal him about (pinks etc)
Made sure I’m off the bridge.
Tried continually moving my character around.

Despite this once he’s around 4 bars he’ll just heal to full health again.

I’ve tried with a 220 solo doc and also brought a 220 keeper along and same issue happens.