I hear you. Happened to me and a friend on multiplayer SEVERAL times. It is frustrating. We were in 1.05 when it happened though. And yeah, we concluded it seemed to happen just sometimes when one player was killed. The 2nd time we fought him, my friend died, and I slivered him to no health, then I died. A few more hits on him, and we may never have known the frustration. Then we fought him for 3 or 4 hours where most of the fights included him healing to full health. So frustrating.
Either way, I'm not sure if this was intentional or not.
We found that the only way to defeat the Lambach was to try and kill him as quickly as possible. I mean, that's kind of a no-brainer, I guess, but we spent a lot of time killing adds and hanging on to our aspect transformation conservatively, for when we need it. We decided that the longer you were in the fight, the more chance he would get that mystery heal in. So, we changed the game plan.. dump everything we got on him, including aspect transformation, when he's in that phase where he pukes up spawn. You may need someone to help with the fight perhaps too. But yeah, that was a real bitch.
After the first day of several hours of trying to kill him, we quit the game. The next night, we tried our new gameplan, and I think we had him dead in maybe a few tries, tops.
I posted about this too here: https://wolcengame.com/forum/bugs-issues/bug-reports/bug-auto-fully-healing-act-boss/
