Saving and exiting a world in Minecraft 1.21.1 (unmodded/vanilla) will freeze the game, it feels like the exit-world mechanism has a deadlock as it won't stop showing "Saving world" in the game.

Temporary Workaround: Kill the process to restart Minecraft.

Yes, the world seems to be properly saved when killing the process although the game still shows "Saving world"

Yes, the minecraft client is unmodded/unmodified.