Finishers should count as abilities whenever tasked to get kills with abilities, no matter the source.
And fix the darned bug about enemies dying before the finisher can get used causing what appears to be a random death upon using a finisher. All finishable enemies used to go immune once they were in a finishable state, so in theory all you need to do is roll back whatever changed that.
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It started this season, so I'm guessing it's related to a seasonal mod somehow. It will hopefully disappear in 2 weeks. There was a similar scenario in a previous season.