(I had to place this in the wrong Version, since if I selected bedrock it would just send me to dedicated server for no reason.)
When you create a world in minecraft preview(used Version 1.21.20.24) and copy it into the bedrock world folder(1.21.2.2) and join the world, unknown blocks are all across the world for no reason, since at the place they would have been in the preview version there would be air.
Downgrading worlds is not supported.