This has been happening since 1.13 snapshots.

What I expect to happen: I go to a chunk that is generated but not populated and the game populates the chunk (generates ore, grass, etc.) on the terrain that is already there.

What happen: I got o a chunk that is generated but not populated and the game completely regenerates it from scratch.

I specifically pre-generated -5120 , -5120 to 5119 , 5119 in Beta 1.7.3 (yes, the world is that old) but did not populate so I have old terrain (and therefore no sharp chunk edges) within that range but new features on the old terrain. Until the 1.13 snapshots, this worked perfectly. Then it stopped, creating a mess on the world.