I was loading up my world, having left the game in the nether the previous time (nether wastes biome). To my surprise, an overworld track (either Floating Dream or Comforting Memories) started playing there. Happened to me 3 times by far, and every time it was right after the world load. The sound event seems to be the generic music one.

Here's the video:
https://youtu.be/xdx5pA-_pmE