Reproduction steps:
1. Open the attached project;
2. Open the scene "Main";
3. Click on the Main Camera and enter Play mode;
4. Observe the Camera Preview window (without_playMode.gif);
5. Exit Play mode;
6. Click on Main Camera -> Clear Flags and change it into Skybox;
7. Enter Play mode and change Clear Flags into Don't Clear;
8. Observe the Camera Preview window (in_PlayMode.gif);

Actual result: when Clear Flags is set to Don't Clear without Play mode, the texture becomes black or white, when you enter Play mode. When Clear Flags is set to Don't Clear in Play mode than the texture render the image as expected.

Expected result: the texture should render the image, when Clear Flags is set to Don't Clear without Play mode.

Reproduced with: 5.3.7p4, 5.4.0f1, 5.5.2f1, 5.6.0b10.