UWorld::CleanupWorld causes issues with sublevels when changing maps after building lighting

A licensee has reported that UWorld::CleanupWorld is causing issues with assets in Sublevels if you build lighting on the parent level, save all of the maps, and then change to another level. The assets seem to be moved erratically (or disappear entirely, as in the repro above)

Regression?: Yes
This did not occur in 4.20.3 Binary

1. Download the attached project
2. Open the project
3. Press the "Build" button to build lighting
4. When Lighting is done building, press File > Save All and take note of the two cubes in the level
5. Open EmptyLevel
6. Open MainLevel
   Result: Both cubes, which were in the two sublevels, are missing (you should still see their shadow from the previous light build)
   Expected: The cubes are still visible.