Steps to repro:

1. Download attached project: https://drive.google.com/open?id=1Kbhgw2K8Zv_nJbkpNOr_DhyHQUmPVn6H
2. Open 'Lamp' scene;
3. Launch Play mode.

Expected result:
Lightmap data is preserved on renderers.

Actual result:
Lightmap data is lost after launching Play mode, so there is a clear difference between meshes in Edit and Play mode when lightmapped objects contain nested prefabs. See attached video.
Regression introduced in prefabs/improved branch.

Notes:
- Reproducible only in prefabs/improved branch;
- Not reproducible in 2018.3.0a4 (c236e17e8c39) , branch: trunk;
- Reproducible on Windows and OSX;
- Reproducible only when the lightmapped object is either a prefab having root in the Project folder with child gameobjects, or a nested prefab;
- Reproducible only in the Play mode; Not reproducible in Standalone Player.