If you die in a different dimension you can teleport to your grave. If you do that, and then while the game says "Loading Terrain" immediately close it/disconnect, you will be invincible when you rejoin. It looks like this persists until the next time you teleport to a grave.

After further inspection, the Invulnerable tag seems to be set to 1 in the player NBT data file. This can be changed to 0, removing the invincibility.