Thank you for reporting! I can confirm that this is indeed a bug and not by design. We are currently in the process of reviewing a fix for it. We appreciate your patience as we get it sorted out!
Edit: Apologies, it looks like I mispoke and this is by design. We’ll be updating the tooltip to reflect that this only triggers on kills by hit, not DoT.
Please also investigate tweaking this broken perk. muskets can just stand on walls on defense, easily build 15 stacks to do 30% more damage. Needs to be either 1% per stack or each stack you build only lasts a few minutes.