I was on the Nether Roof when the vanilla server I play on did its bihourly restart, and upon rejoining, an Overworld ambient song was the first one to play.

To reproduce:
1. Join or start a vanilla server.
2. Go to the Nether (might only happen on Nether Roof).
3. Restart the server.
4.. The first track that plays will have a chance of being an Overworld one.