If before your able to respawn you are given health boost the game refuses to allow you to respawn, upon exiting and reloading the world you are put in a limbo state of both dead and alive, being only able to interact with blocks within a certain area. In order to leave this limbo state you must deactive the health boost and kill the player.

Steps to reproduce

1 Create a repeating command block with the command

/effect @a health_boost 10 10 

2 Die

3 click respawn

4 observe softlock

Expected result: being able to continue to play minecraft without leaving, rejoining, disabling commandblocks, and dying.

Actual result =!"Expected result" and being stuck

Activating immediate respawn removes this issue but you shouldn't have to do that for health boost to work properly.