Seamless lightmap bake on large surface?

I've got a bit of a problem with the way unity does lightmap baking. I tried making a test scene with 4 different planes just to figure out how it works.

• Plane01 is just a normal polygon.
• Plane02 is 4 polygons.
• Plane03 is 4 polygons separated inside the mesh, they aren't attached to each other.
• Plane04 is 4 polygons where each polygon has been detached as a new separate object.

All the planes except Plane04 has the same scale in the lightmapper, because of the way it automatically unwraps imported assets. Which I find a bit weird, Plane03 should in my mind work the same as Plane04, since nothing is attached to each other.

But okay, I now know that it doesn't, but if I use Plane04 and proceed to bake a lightmap, I get pretty obvious seams.:

I have quite a few large surfaces, and if I don't find a proper solution, the only fix I can think of is manually editing the finished light maps once baked.

Any possible work around to this problem?