Was going to report this as well, but saw this has been an issue for a while now. Hoping to bump this, because its such a good item for her otherwise.
Through my testing I found that if her ult doesn't kill, but the burst itself tips it over the line (improved or not) you do not get granted the soul bonus. I'd like to know if this was intended as well
Edit: the damage breakdown will show the "burst" as the "killing blow" which must be the determining factor for whether the souls are granted for the kill or not