After switching to using the Game Manager, I encountered a unintended consequence:
the game from before the switch is still loaded in the cache of my players, so they can log in and manipulate stuff (even add scenes if they have permission) on their own copy, but are not actually in the running game. Outside the fact that those players cannot be seen as online (and they in turn can’t see others as online), the copies behaved exactly like the regular game. This happened to multiple players, all on current firefox (83.0), and could be resolved by clearing their cache stored under the previous url. This of course means all changes on the individual world copies were also deleted, but no other ill effects were encountered afterwards.