Example: https://drive.google.com/file/d/1xewltZaVQ9CMfMFu4XCeILOEqtLbLjuj/view?usp=drive_link

This flat world was first created in 1.15 where the old bedrock level was y=0, and has since been updated up until the current version. Blocks that I've placed in the void (above y=-64), then destroyed, reappear whenever I log out and re-login.