Trail master regen/Invincible

decided to run some ultimatums and got the trial master. Hit him once or twice and as soon as he hits 75 percent health he teleports away and instantly recovers all health and energy shield. and becomes invulnerable for around 5 seconds. This repeats about 10 times until I finally fail. the regen was so fast I cant out damage it, and he is invincible on top of that too.
There is nothing listed on your own website about him recovering his entire health and becoming Invincible. Am I missing something? Cant find this in forums.
I wasn't even running a badly rolled map either.

Are you a DoT build?

Did the map have patches of Consecrated Ground? Did you take the Ultimatum modifier where you do no damage for 3 seconds every 8 seconds?