New player, how does Necromancer trait work exactly?

Kinda new player here, I had a question about the necromancer trait.

So today me and my buddy downed a solo hunter and I shot a flare at him. While he was burning, we got distracted and about 30 seconds later we noticed that his body was gone. Shortly after that he began shooting us again.

Then we killed him a second time and I shot another flare at him, this time watching his body for about 1 minute to make sure he was dead for good.

I thought that once a downed hunter was on fire, he was unable to resurrect himself, but apparently I was wrong. So how does it work exactly? After the match we made sure to check the details and he really was a solo hunter, so nobody helped him put the fire out.