Burning Dead interaction with Frostburn?

So I’m doing a Frostburn Trickster and when I started leveling it, I went for Lethal Assault first because it gives a ton of early damage without an energy cost. The point being, I only had 1 point in ABB, so I didn’t have much frostburn damage yet.

However, once I got to Burrwitch Village, I had enough devotion points for Tsunami, so I actually had some frostburn damage.

I started noticing that the Burning Dead sometimes wouldn’t explode. When I let them die to frostburn, they seem to never explode, but they will when I kill them with my standard attacks.
Has anyone else noticed this? Also, I’ll post again once I get to Act 2 and see if the Arsonists explode or not.

I can’t make it happen consistently, maybe it’s not the frostburn. Either way, it’s happened multiple times again, so either it’s a consistent bug or it’s a feature that I don’t know the reason behind.

If it’s not bug but game mechanics - it’s perfect news, that burning zombies on ultimate explodes like a mini-nuke bomb.