The shader turning black in the preview is pretty random. I've had it do weirder things...
I will be switching this to use a custom shader solution, similar to the animation editor - this will enable scrolling the tilemap, etc.
Until then, you could hack in a solution by going to tk2dTileMapBrushRenderer.cs, look for dictData.materials.SetPass(); - instead of setting this material, create a new material, set mainTexture & use that instead. That should fix it for you.
Let me know if you get stuck and I'll post some code when I'm back at my computer.