[Shujin]

Sometimes when you beat the boss, and wait for the next spawn while grinding in the place, it still counts you are having participated.

so you can not do the boss in the second spawn right after.

[FaeLenx]

I believe this happens if you engage in a boss fight, the boss despawns from the room, and then you beat it after it's already gone.

[Snake]

Yeah, the proposed fix is for the boss fight to count as if you fought it the moment you initiate the fight, not after you kill it.

Pros: Prevents this.
Cons: Prevents a rematch in case you lose.