Grass bakes with some meshes randomly very dark

Without and baked lighting data my grass meshes look fine, however when I bake some look as expected and some bake really dark. There is nothing around to influence their shading, so I'm really confused why they bake in such a variety of extreme dark/light shades.

The normals of the grass cards are fine, they are set to static and have enough padding in their lightmap UVs.
I can't think of what might be causing it.

Is this a know issue?

 

I would make them be lightmap static but not use any space in the lightmap ,then you can use Lightprobes to light them.
Saves baking time, lightmap space and Artefakts.
Yes i had a lot of unexplainable errors with light-mapping of small detail or complex geometry.