Regenerating terrain in (unloaded?) chunks

Terrain regenerates itself when going from a pre-snapshot save to a snapshot file. The old buildings above ground are (with a few blocks missing or replaced) kept. I've tried recreating this bug twice on both snapshots, and I've tried it on 1.13.2. On 1.13.2 it shows no problems, but as you can see from the screenshots, it does have them on the newer versions.