After entering the Lost Colosseum subworld, beating Bereft Vassal for the first time, and coming back to the overworld, I found that several of the boss entries in my Boss Checklist were marked incomplete even though all of them were beaten. I downloaded this mod afterward, but the bosses were still marked incomplete. No big deal, I killed them all again and my list was complete again.

But for some reason, even with this mod installed, the Bereft Vassal will not be marked as complete, no matter how many times I defeat them. I even entered and exited the Lost Colosseum through the gateway itself rather than using the Waterglass Token to warp there, but it's still marked incomplete.

It's not a huge deal since I can just manually check it off, but still.