I had a 1.16.1 world called new world. When I launched Minecraft in 1.8.9 to play around with floating sand, the world did not show up on the save list. I created a new world (in 1.8.9) and left it with the name new world. I didn't realise it at the time, but this overrode the blocks in the 1.16.1 world but left my inventory the same. My original 1.16.1 world is now gone and the folder in the saves directory has been replaced by the 1.8.9 world.