When you're fighting Nechryaels, they summon some Death Spawns. The problem is, this isn't a multi combat area. So in this case, when the deathspawn attacks you, the Nechryael stops attacking you, and then the death spawn despawns. The same thing happens when you switch to attack the death spawn instead of the Nechryaels.

I can grab a video if you need it.

Thanks,
Cody