Additional food for thought:
As described, the current game behavior is a bug because damage was dealt, even though the total damage was more than sufficient to kill the boss, and the players in question were in battle. Clearly the health was not “ticking down as players from the Kingdom deal damage to it” otherwise we wouldn’t have 4 players with total damage equal to the bosses health. The leaderboard is clear evidence that the damage was dealt, regardless of whether their specific damage resulted in the boss’s health being reduced to zero. (I’m sure there is a “well how was this programmed” element to it, which is why it seems to me to be a bug).
This may be less of an issue once the Boss Tier is high enough so that it takes many player attempts to kill the boss, but as it stands right now, it appears that if two or more persons are in battle when the boss dies, only one’s damage is being counted for purposes of the individual silver/gold awards.