Steps to reproduce:
1. Create a new scene
2. Set Environment Lightning Source to Color and Ambient Color to white (FFFFFF)
3. Open Project Settings and in Prefab Editing Environments set this scene as Regular Environment
4. Create new 3D Object -> Cube, make a prefab out of it and start editing it in Prefab Mode

Expected result: Ambient lighting doesn't change.
Actual result: Ambient lighting changes to black color.

Reproduced in: 2018.3.0a6, 2018.3.6f1, 2019.1.0b3, 2019.2.0a5

Note: Only reproducible with Prefab Mode which was introduced in 2018.3.0a6.