When I clear a large area of ​​blocks, save and re-enter the world, it doesn't work.

This is a new map I'm building, and I want to put border blocks on the bottom to make sure players don't cross areas that don't have blocks.
I used the command to first lay out the entire border block, and then clear the middle block to form an enclosed area.
But when I saved the game and re-entered the map, a problem occurred in the video. I tried many times with the same result.
By the way, the height there is y=-64.