Invalid texels are not treated the same way in CPU and GPU lightmapper:
When a ray (launched by lightmapper baking) hit a backface we increment a invalid texel counter. In the compositing stage we then try to fix invalid texels.
The bug is that we do not have the same behavior between CPU and GPU lightmappers.

From what I saw in the attached repro project:

-GPULM tries to filter invalid texels with neighboors valid ones hence the distorted shadow.
-CPULM doesn't seem to do anything with it.