I had it happen with Phogoth, and I think I know what triggered it in my case-

I was attempting to do the nightfall, and as I was hiding back in the entrance chamber, I ran out of ammo and needed more.

So I ran back to the hell mouth area right before, got what I needed.

I came back to phogoth's chamber and no one was there- not him or any enemies.

I ran around for a minute, and then he randomly just appeared in the middle of the arena, with all the knights and I died :(

I had him down to about 1/3rd health, and when he reappeared, it was full.

- No text -

Yes, the Winter Kell mission, I think.

That's the result of connection issues and/or sever issues.

I'd guess the obvious, you killed it the instant it was teleporting. It did a check for health from a register and found nothing. Default = Full health.