Happened to me with sepkis.
I was dealing with the adds, hadn't fired a shot at sepkis for a few seconds then he died.
I suspect that if you don't land a hit on the bounty target within a few seconds of his death, you don't get the kill credit.
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Yup this was the first time I've had it happen. someone else just said if the last thing you shot before the boss dies wasn't the boss you don't get credit.