In the 1.18 test, my file got ruined.

I made a superflat world with a lot of woods, stones, iron...etc.

The version is 1.18 test,

When I open it after I update the version, the grass blocks under the blocks I build disappears. 

And when I open another normal survival map (the mode is 1.17), the map has been gone.

Well, I wish you can fix it!