When a world generated from 1.13 or more is outdated, the world instead of eliminating the related changes, in these more current versions, regenerates the world of 0 eliminated all changes made by the player.
This bug affects the versions:
1.13 and / or +
1.14 and / or +
1.15 and / or +
20w06a and / or +
While not exactly the same, I will mark this as a duplicate of MC-45009. Version downgrading has never been supported.