How to reproduce:
1. Create a new 3D Template project
2. Open the Sample Scene and delete the Main Camera and the Direction Camera
3. Create a new camera called "Main" and set its Depth to 1
4. Create a second camera. Name it "Low" and set its Clear Flags to Solid Color
5. Duplicate the "Low" camera
6. Disable one of the Low cameras

Expected result: Game view continues to render the Skybox
Actual result: Game view is rendered in solid color by one of the lower depth cameras

Workaround: Adjusting any of the Main camera's options seems to resolve the issue

Reproducible with: 2020.2.0a16, 2020.2.0b1
Not reproducible wtih: 2018.4.27f1, 2019.4.10f1, 2020.1.4f1, 2020.2.0a15

Notes:
1. If the issue does not occur, re-enable the low camera and try disabling a second low camera
2. Multiple lower depth cameras are required for the issue to occur