The bug
When setting a very high respawn radius on a superflat world and the players dies sometimes they are unable to respawn or even go back to the main menu. The game just gets stuck on an infinite screen saying Saving World and the only way to close the game is by using task manager.
How to reproduce
Make a void flat world
Type
/gamerule spawnRadius 30000000
/kill
yourself
Note that this works as intended on normal worlds.
Can confirm in 20w49a.