Gonna take a guess on this one: I suspect that, on hitting zero health, Harpies trigger a different function than normal - specifically, one for the "Harpy Blast" ability, rather than the standard death function. As such, when the death function actually triggers, it probably pulls the cause of death as the Harpy Blast rather than the attack. The structural way this works could vary a great deal (for example, it could pass the pair [Harpy, HarpyBlast] in a data set), but the basic idea here is that the game reads their self-destruct as their cause of death.

Somtimes, enemies means those with legs. Happens with melee 30 without dying bounty as well.

I made a subject about this before, but it was buried. Most things that trigger upon killing an enemy don't work on harpies, both weapon skills and class skills (I was using hunter.)

And here I thought it was just me