Depth not working properly when using custom materials

When a mesh is passing an adjacent or intersecting mesh the order seems to change. 

Observed:

• The point of flipping seems to be affected by the camera angle.
  • camera angle close to 0°: Order flips at the middle of the passing object. (like on the screenshots)
  • camera angle close to, but below 45°: Order flips immediately.
• The issue disappears per axis when using a camera angle > 45°. 

The provided project has 5 cameras with different angles to make the issue more clear.

On the screenshots:

• Left side: QT build in material
• Right side: custom shader (just setting fragOutput to color property, nothing fancy)