It’s been 3 weeks, no new information provided by @Jeto. I think we can assume at this point that the event is “working as intended” in that players who are in combat when the boss is killed effectively lose their attack and any scoring that had been done in that combat up to the point the boss was killed is lost. At this point, whether its a bug is somewhat irrelevant as a “fix” would just introduce the silly consequence as discussed earlier of constant coordination to have multiple people in combat when the boss is killed for “credit”.
Instead, why not just make the event better! Here’s some suggestions how that can happen starting with the introduction of an “overkill mode” that essentially renders this entire thread obsolete: