Was doing scalecaller and fang lair farm and this would only happen between 100% health and 85% health, the boss would disappear and completely reset. It happened on 4-5 bosses.
Im not 100% sure its the betty netch but i took it off for the last run we did and had 0 resets. The reason why i think its the betty netch is because its my first time doing it on warden and it has never done that to me on any other toon.
I think its worth taking a look at