Chunks disappearing after shutting down and starting up world.

Hi,

Since two days ago a friend and me decided to start another, own made world in worldpainter, to upload in 1.16.5 with forge and worldedit installed on aternos.

We uploaded the world and played in it. We noticed after shutting down the server and coming back to play, everytime there is corruption in the world. Chunks and huge parts disappeared and this world is really small in size. It stays like that singleplayer.

When starting in singleplayer , it doesn't happen.

How can we find the cause of this and how to fix/prevend it.

Two years ago we made a another world in same way but older versions, now its in 1.16.4, we don't want to update it since it gives us so much trouble with a small world like this.

I tried the blankworld method, unzipped method etc. Still happens.

Its not lag i think its permanently gone , i could fill up the space with worldedit. But thats stupid work if it always is gone everytime

Didn't work

that one is standard turned off

I found the problem, aternos turns on the optimize button on automatically while i disabled before.
This is so sneaky, so i need to turn this off every time? And when i disabled it while starting it suddenly enables it on itself, so i need to refresh every second while starting it....

well it works for now so i think this is solved, thanks for helping!