So I'd have a diabolist with three souls. I'd kill something using the explody power, it would show the soul gathering animation from the newly dead thing, and afterwards I'd have one soul instead of the two that I'm supposed to.
Unless there's something about diabolists not gathering souls that they themselves kill, this is a bug. I've done this killing war elephants and... probably a couple other things. Don't remember specifically.
This will be fixed in the next update.