Reproduction steps:
1. Open the attached project "BugSubmission.zip";
2. Open project to “Menu” scene;
3. Press “Load Scene” to load into an example geometry scene (lightmaps are applied as expected);
4. Press the “Next Config” button to switch lightmaps;
5. Press “Return to Menu” button to return to the menu;
6. Load the scene again.

Expected result: the same lighting will be applied after re-entering the scene which is loaded additively.
Actual result: All static objects are drawn white, only dynamic objects still receive directional light and ambient from probes.

Reproduced on: 5.6.2p4, 5.6.3p2, 2017.1.1f1, 2017.2.0b10, 2017.3.0a7.
Not reproduced on: 5.6.0f1, 5.6.2p3.
Regression firstly introduced on: 5.6.2p4.

Note:
- Not reproduced on the lower than 5.6.0f1, because some of the properties like "shadow mask" and "lightmap color" were introduced with this version.
- If scenes are not loaded additively, the issue does not occur.
- If lights are only set to baked, the problem doesn't occur