How to reproduce:
1. Open the attached LightmodeWarning.zip project
2. Open DirectionalBaked scene
3. Make sure that SceneLoader (In the Hierarchy) is having UI scene for the additive load (In the Inspector) and 'Make Added Scene Active' set to true
4. Enter Playmode and observe the lost smoothness
5. Uncheck the 'Make Added Scene Active' and enter Playmode
6. Observe smoothness has persisted

Expected: Smoothness has persisted
Actual: Smoothness is lost

Reproducible with: 2020.1.14, 2020.2.0a16

Note:
- Could not test with earlier versions because of the thrown error:
Failed to load '/Users/vita/Downloads/case_1259779-LightmodeWarning (2)/Assets/Scenes/DirectionalBaked/LightingData.asset'. File may be corrupted or was serialized with a newer version of Unity.