Making a New World on a different version

Not sure if this is a copy, but I have a world in 1.16.1, which I play with my friend. When I switched back to 1.8.9 to play on a different server, I decided to make a new world to practice my building skills. But when I hit singleplayer, there were no worlds available, not even my 1.16.1 world. I ignored this and made a new world. When I joined that world, I realized I logged into my 1.16.1 world. I switched back to 1.16.1, and when I logged on, all of my buildings were broken up into chunks, in the shape of squares. And when I checked if I had joined the wrong world, but there was only one.