Shadow aliasing artifact stripes can sometimes appear if directional light angle is set at 45 deg angle to a surface with progressive lightmapper.
The case is somewhat rare, but always repros with a specific setup. The artifacts can disappear if pushoff is increased to 0.1
Material is set to 2 sided and there is no invalid texels.
Perhaps a precission issue at grazing angles, and quite rare in most real scenarios, could be considered by design or cosmetic.

1. Open the attached project 'PVRAliasingStripes45deg'
2. Load the scene 'pvr aliasing stripes'
3. Let Auto Generate complete the bake
4. Observe Aliasing stripes in the baked shadow cast by the vertical plane

other things to try:

Rotate the direct light 1 degree on any axis
Turn on/off filtering
Move the watertight game object slightly on the x axis and see artifacts disappear sometimes
Switch Lightmap parameters to 'LightmapParameters High Pushoff' and obersve artifacts disappear
Switch material to 1 sided on all meshes to see texel invalidation

tested with Unity 2017.2, 2017.3, 2018.1