In 1.17.30, when loading a flat map created in a version lower than 1.17.30, all the loaded chunks of land disappear and the Y coordinates of the buildings go down and some are cut off. . This is a very fatal bug, so we hope the Mojang team will fix it soon.

The land disappears and the Y-coordinate of the building goes down excessively and some parts are cut off

On a flat map, the land is the same