I have an old Xbox 360 map which I have tranfered down the line to each newer console and I am still working on the same map now on Xbox Series X so now on this old Super Flat World I can work and build stuff under the map to -64. This issue only seems to happen at or near -64. I first encountered this issue when I accident made a big hole in the map and water started leaking to the bottom at -64 that was covering something else under it so I used some sponge blocks to mop it all up. I then /fill an area with the sponge blocks, and then /fill the same area with air to clear the sponge blocks but when I saved and came back to the world all the blocks I had removed had came back. I am experiencing this issue is many areas of the map all at or near -64. I have a onedrive link below of a video showing the blocks coming back.

https://1drv.ms/v/s!AgjMO8uuoSNz-yk4D5oENpVlQjOK?e=907TGs