To reproduce:
1. Open attached project;
2. Open "test" scene;
3. Make sure Allow HDR is enabled on both "Main Camera" and "TestCamera";
4. Open Test Window (Window > Test Window);
5. Press "Start Test";
6. Move around any Cube (by changing its Position in the Inspector Window).

Expected result: Background of the TestCamera is cleared when Clear Flags is set to Depth only.
Actual result: Background of the TestCamera is not cleared when Clear Flags is set to Depth only.

Reproduced in: 2017.3.0a7, 2017.3.0b7, 2018.1.0a2.
Not reproduced in: 2017.3.0a6, 2017.2.0f3.
Regression introduced in: 2017.3.0a7.