Lightmapping Artifacts With Combined Objects In Unity 5

I am experimenting with baked lightmaps in Unity 5 and combined objects. There are artifacts on the corners of some of the objects. The scene setup is:

• 8 default cubes scale ranging from 1 to 2.5 and a plane

• 2 point lights set to Baking=baked and soft shadows

• Lighting -> Baked GI enabled

With all the cubes as separate objects, the scene looks great! When I combine the cubes into a single object to reduce drawcalls and re-bake the lightmaps, there are artifacts at the corners of some of the cubes.

Here are my lighting settings:

I have tried:

• Combining objects with no UV2 channel

• Combining objects with a generated UV2 channel using Unwrapping.GenerateSecondaryUVSet

• Combining objects, exporting from Unity to Blender. Generating a UV2 channel with lots of padding (10 pixels per island) re-importing into Unity.

• Tried everything with and without the "preserve uvs" checked

Nothing seems to work. The tainted corners are always present. Any ideas how to fix these?

Thanks.