Reproduction steps:
1. Open the attached project named Case_1200736
2. Select the Terrain in the Scene
3. Select Paint Texture Tool -> Create a new Terrain Layer
4. Observe the Terrain

Expected result: The newly created Layer gets applied to Terrain as a base layer
Actual result: The newly created Layer does not get applied to Terrain as a base layer

Reproducible with: 2019.2.0b10, 2019.2.12f1
Not reproducible with: 2017.4.35f1, 2018.4.13f1, 2019.2.0b9, 2019.3.0a1, 2019.3.0f1, 2020.1.0a14

Fixed with 2019.3.0a1 and above

Notes:
- If a second Layer is created, then it will be applied as a Base Layer of the Terrain