System Information
Operating system: Windows-8.1-6.3.9600 64 Bits
Graphics card: GeForce GTX 750/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 388.13

Blender Version
Broken: version: 2.80 (sub 72), branch: blender2.7, commit date: 2019-05-26 22:34, hash: rBd525c76003b3
Worked: (optional)

Short description of error
Overlay, subtract, multiply and divide Layer Blend modes in for the Grease Pencil don't blend right. When I draw a white line in Overlay Blend Mode over a 60% grey area, some areas of the white line become dark instead of losing opacity (see image attached). When adjusting the opacity, the problem becomes more visible and the stroke flickers when the opacity is close to 0.0.

1. Start a new 2D Animation scene
2. In the material tab create a grey fill material with a color value of 0.6.
3. In the material tab create a white stroke material with a value of 1.
4. Draw a simple square shape in the Fills layer with the Grey Fill Material.
5. Draw a few lines withe the White Stroke Material, on top of the square in the Lines layer.
6. Change the Blend mode of the Lines layer to Overlay.
7. Change the Opacity of the Lines Layer to different values including 0.0, 0.01, 0.5, 1.
8. Observe the white line in the view port.

NOTE
This problem shows in render too.
Blender file attached.

Thank you!