I have also seen this where the combat event and the kill feed report different killing blow information. It is quite odd. In PvP Alerts I synthesize the Killing Blow information returned by EVENT_COMBAT_EVENT with the Kill information from the kill feed event to assign killing blow skills to the kill feed data and have seen this bug result in players getting killing blows with skills from a completely different class.
I have also seen events in the kill feed where two different players are credited for the same kill.
Something is very wonky in the back end.
I also posted in Wish List when the Kill Feed first dropped asking for KB skill info to be added to the Kill Feed API in part to mitigate this issue:
https://www.esoui.com/forums/showthread.php?t=10677
That said, as Baertram said in the other thread, based on the intended order-of-operations in game mechanics, the NB heal would seem to be working as intended.