I was doing a casual playthrough of Guest Star mode as Magolor, and I was in the room before the Flamberge fight in the fifth split. I went into the bonus room (the one with the meteors) and cleared it normally. I then accidentally re-entered the bonus room, then left through the entrance. Instead of reappearing outside the bonus room door, I appeared in the Flamberge fight. Though I haven't done all the testing I would like to yet–I haven't yet tried to replicate the glitch where it originally happened yet–I tested the bonus room in the room before the Whispy and Yggy fight (the one with all the Comos), and have not found a way to replicate the glitch there yet. (In fact, this is the reason I have not yet tried to replicate the glitch as it originally happened; I have been doing the tests with the Como room and I haven't gotten past that yet.) I've documented all of my attempts to do so, if you are interested. My question is, is this a known glitch?

As far as I know that is not a common glitch but I am not an expert on these things. I could take a look into it and try myself though. I did not speedrun the game yet, but I have beaten it multiple times and played guest mode. I never encountered this glitch.