Reproduction video, several outcomes depending on when world was created/re-created/entered:
https://www.youtube.com/watch?v=-On-BvTGX24

I created a default grass flatworld sometime in the 1.13 snapshots (later than 17w47a), and because I wanted to test something in an older snapshot version, I re-created the world in 17w47a, but fell into Void upon entering the in 17w47a re-created world.

As you can see in the attached video, the Biome is still Plains, but there are no blocks, as the game didn't convert it when I re-created that newer world in an older snapshot.

I would have expected the game would convert world types properly, considering I did not try to open an existing worldsave from a recent version into an older version, but that I just simply wanted to re-create that world type in an older version.

But I'm not sure if this can be considered a bug or "WaI"/"Invalid", I'll leave that up to you to decide