Steps to reproduce:
1. Open the attached project
2. Open the SingleCameraRepro scene
3. Observe a camera with clear depth and reduced viewport does not clear depth and renders on top with a black background
4. Open the MultipleCameraRepro scene
5. Observe a camera with clear depth and reduced viewport in between two full viewport cameras gets overdrawn
6. Open the Player Settings -> Other Settings, add GLCore
7. Open the SingleCameraRepro scene
8. Observe the issue in game view
9. Open the MultipleCameraRepro scene and enable "Camera Depth 2 (Full Viewport)"
10. Observe the depth 1 camera get overdrawn

Actual results: Camera with clear depth is rendered in wrong order with GLCore
Expected results: Order should be same as with DirectX11

Note:
Not reproducible with Metal and DX11.

Reproduced with: 5.6.2p2, 2017.1.0f3, 2017.2.0b3
Not reproduced with: 5.4.5p4, 5.5.4p2, 5.6.2p3