I'm in a new singleplayer world in the newest snapshot (at the time, 18w44a), every time I die in the world, I click "respawn" and the entire world is gone, I have to save & exit, then rejoin the world just to get it back.
Environment
My operating system is Windows 7 (64-bit with 8GB RAM)
Java version 8 update 161 (build 1.8.0_161-b12)
This is not a complaint or anything, it's just to inform you..
It might not even be the version of the game that's causing it, it might just be my computer, I can't really test that theory.