This a tricky bug to describe so bear with me.

After a player dies in the nether (the way I died was I fell into lava from a high distance), the player will successfully respawn in the overworld (original spawn, no spawn point that was reset, and no bed), but after running a short distance, will unexpectedly be killed by the game.

It happens repeatedly without a cycle, so all it takes is one death in the nether.

Walking in the overworld from the spawn point is fine, and so is jumping, but after a few moments of running, the player is killed.

I don't know if this has something to do with time, or if it has something to do with distance. It could be that a player is killed after a specified about of seconds once the player has began running, or it's a certain amount of blocks that the player travels before being killed, or both.

Note: I was flying in creative mode in the nether, then I switched to survival mode mid flight and fell into the lava (purposely) before the bug had occurred.

Also, I tested this in other new worlds, and it was the exact same experience.