When killing one of the bosses with Erasing Strike, they do not leave a Nerotic Spirit, where they died.
I am not 100% sure if this ins intended or not.
I also just did this fight once and only one of the bosses died through the Erasing Strike hit.
The other 2 bosses i killed with Rive and they did leave a Necrotic Ghost behind.
Here is a video:
Here my ES Skill Tree:
https://imgur.com/XbcggNx
Here my Player.log:
Player_Spirits of Fire Erasing Strike.log (1.1 MB)