World played in 1.16 Snapshots replaced by some kind of chunk error

Earlier this year, I worked on a superflat world to try to build a server layout. At that point, I was waiting for 1.16 to release so I could open the server (I created the world in a snapshot). Eventually, I gave up on this idea as I had other things to do. I did not upgrade the world to 1.16 when it came out. Today, I was searching through some saves on my PC as I had to delete a few (my PC is running out of memory), and stumbled across the server world. I wanted to open it to see how my building style has progressed, as it has gotten a lot better over the last year. When I clicked to open it, it asked me if I wanted to make a backup. As I usually do, I did. Then I opened the world. When I loaded the world, I was stuck in a huge dirt cube. I changed my gamemode to spectator, and flew out. The superflat world had turned into something reminiscent of the "Far Lands", but without the holes. Grass had begin to grow on top of this huge wall, and on the other side was a taiga biome. I have no idea why this would happen, but there was no trace of my buildings anywhere. I will upload screenshots soon. Also, may I note, I could not create maps in this world before, but now can.