Chunks marked for repop are recreated from scratch when upgrading from 1.12

When a 1.12 world is upgraded to 1.13, chunks marked for repopulation are deleted and regenerated from scratch, as far as I can tell.

Expected behaviour

Existing structures in the chunk marked for repopulation should be preserved, and population is added on top of it. For example, if you built a house with a grass roof then marked the chunk for repopulation, next time you enter the area you might expect to see grass + trees on the roof. This was especially useful for larger areas.

Actual behaviour

The chunk is deleted and regenerated from scratch. For example, if you built a house then marked the chunk for repopulation, next time you enter the area the house will be gone.

Steps to reproduce

1. Load the test world (attached) in 1.12.
2. The pretty building is still intact, and a chorus flower has grown on the roof
3. Load the test world in 1.13.
4. The pretty building has disappeared
5. Note: you have to load two copies of the test world or the chunk will no longer be marked for population