The game always seems to think that there’s an extra player, even when I’m playing by myself with a controller. This probably has something to do with playing on a Steam Deck. My guess is that it sees the Steam Deck (as a controller) and my wireless controller and assumes both are in use when really I’m only using my wireless controller.
To reproduce, connecting multiple controllers while only ever touching one might work the same way. But to more closely match my situation, use a Steam Deck, connect a wireless controller, and play a level.
This is in v24.6.1, I think- whatever the latest alpha version is at the time of writing. I’ve experienced this in older alpha versions (dating back to when the editor alpha channel existed) but I never experienced it on the stable branch.