Flat world broken when i join

I create a flat world I go in and the ground is ok then I leave and when I join back I see everything destroyed grass blocks , it is not possible to play like this, it is the 100 time I create a flat world and I find it like this, I cannot use the fill command to fix things because every time I repair the damage then when I come back I find it worse than before more laundry> and if I don't fix it I find it the same worse fix this bug that I'm bored having to use the fill command and then it is the 100 that I eliminate a flat world with beautiful constructions but pierced by the bug