If the kill was made after the boss died (but before the post-match status screen), this is normal behavior.
I didn't download the replay to check, but I think this is what happened
Thanks, that's fair enough. I don't know if the kill was in that period of time, though I would argue then that this shouldn't be the intended behaviour as it's a bit confusing. Perhaps the kills at the very end of the game after the Patron is killed, just shouldn't be counted. That would be my suggestion.