Repro steps:
1)Open the project and Depth Issues scene in it
2)Disable Camera - Layer 1 game object
3)Drag Sphere - Layer 2 gameobject on X axis back and forth
4)Notice that the depth buffer side of the game view is being cleared, whereas left side - not (look at .gif attached)

Expected/Actual behaviour can be seen in the .gif - depth buffer shouldn't be cleared when the camera has Don't Clear set

Reproduced in: 5.3.5p2; 5.2.4f1; 5.1.4f1
Couldn't test on 5.4 due to case 802624(clears everything)