I created a world in 20w45a and have been upgrading it to every snapshot so far, and in 21w05a I tried using the /setblock command to place cave vines in the world (I tried to replace a barrier block, however the type of block may be unimportant), after that I noticed that the world was bugged and would not save and close.

Using the 'save and quit to title' button just hangs the game, same with F3+C, the only way to close the game that I found is by ending it in task manager.

This bug does not appear to happen in worlds created in 21w05a