Opacity is broken

[Jonas Ruebsam]

I have a scenario where I create a large polygon which shows the current field of view of the sensor.
This polygon should be transparent (so opacity is set to s.th. like 0.5), however in some situations objects behind this polygon are not drawn. See this picture:

Here you can see the green boxes which are partially occluded by the sensor FOV are still visible.
However, the red cubes which are occluded are not visible.
This behavior also seems to be completely random, sometimes red is not visible, sometimes green, sometimes both, sometimes everything works as expected. What could be the issue here ?

Cheers