Last night, I decided to check out 1.15, as I was curious about it and had been playing on 1.14.4 for a while now. I loaded up my survival world that I'd been working on for a few weeks, and created a backup for any bugs that may happen. Everything seemed cool and well, but I forgot that I didn't have Optifine for 1.15, so I just switched back to 1.14.4 so that I could have shaders on and all that cool stuff.

This morning, I got on my world and tried teleporting to my house as I always do, and when I did, it completely disappeared. It looked as though everything went back to before I even made room for the house on the land. I thought it may just be a glitch, so I left the game and re-logged back in, still had the same problem. I teleported to a second base that I'd made yesterday, and funnily enough, that one was still there as if nothing happened at all. I checked my backups folder to see if I could use the backup that I'd made prior to using my world in 1.15, but I couldn't figure out how to use it. After some googling and reading multiple reddit posts and things of that sort, I was still confused and didn't have the same type of backups folder as other people, apparently.

I'm not sure if it'll happen again, but from what has happened already, this is how it came to be:
1. Have a 1.14.4 world
2. Load it on 1.15 & make backup
3. Switch back to 1.14.4, and the last place you slept at will go back as if you'd just recreated your game, and the land will go back to when you never touched it.