When you use a piston with certain blocks on the worldborder, you can merge blocks and create "ghost blocks" once you destroy them, that you can stand on but cannot destroy and is not seen in the debug menu. Blocks will also appear to "leap" when the worldborder is increased by 2 or 3 blocks

I have also attached the world file

From my tests, this does not affect blocks that cannot be pushed such as chests or obsidian

I will link the video here: https://youtu.be/eeSRSPAD53s