Once Triggered Immunity has proc'd, taking a hit that would otherwise deliver damage of the type you're already immune to; which would also proc Triggered Immunity, heals both endurance and health to full.
I'm not certain of my explanation, but it seems to fit what I've experienced.
A separate potential issue with Triggered Immunity is that once it procs for one damage type, if proc'd again for a different damage type, while still immune to the first damage type, the first immunity is cancelled. I don't know if that's an intended mechanic, it just seemed counter intuitive given you get two uses of the ability per encounter.
Thanks!