Creating and loading a new world in Minecraft 1.21.1 (unmodded/vanilla) with cheats enabled will deny me to use cheats (including the game mode switcher: "[Debug]: Unable to open game mode switcher: no permission").

Temporary Workaround: Open to LAN (with again "Allow Cheats: ON") will allow me to use cheats, but ONLY when being in a LAN world.

Yes, cheats are enabled in the created world, I even checked with an NBT editor.

Yes, the minecraft client is unmodded/unmodified.