System Information
Operating system:OSX
Graphics card: RADEON PRO 460

Blender Version
2.80

Short description of error
semi-transparent strokes and fills overlapping themselves create ugly artifacts

Exact steps for others to reproduce the error
Based on the default startup or an attached .blend file (as simple as possible).

Draw a freehand zigzag stroke with the Draw Block Pencil. Set alpha to 0.5.
At sharper parts of the line the individual "dots" of the stroke overlap this create some darker triangles perpendicular to the stroke direction.
draw a closed shape with curvy lines. set fill alpha to 0.5. The fill overlaps concave parts of the outline sometimes.
Add a Noise Grease Pencil Modifier. factor ~13 to see the line points position move a bit , Full Stroke off
The fill overlaps somehow with itself, "folds" and creates darker lines inside the shape.
Now set the material Fill to fully opaque and the Opacity of the Layer to 0.5, trying to make it look better.- No, the dark areas are back.
Perhaps set the Blend Mode to something like "Darker Color" so only the darker color gets rendered- Since there is only one color in this fill, this could make it even (?) No. No Blend Mode like that, and it would possibly only affect the appearance of underlying Layers...

The new Grease Pencil would be a very beautiful tool without this weird alpha behavior.