Chunks Being "Mixed up" When Re-joining Singleplayer World

I went to a different singleplayer world to test something and when I came back to my main survival world, I was suffocating inside a chunk from a mountain that had been moved. I switched to creative to see that the area around me was mixed chunks from other areas around me. The affect existed underground, as well and duplicated a monster spawner but turning the duplicate spawner into a pig spawner. Images below.

 

Sidenote-This is my first ever bug report so Im not sure if I've done everything correctly, but thanks for reading regardless.