Patrick Chase
Was the sandpaper you used stearated ("no-load" coating)?

What you describe sounds like what I would expect if the stearate coating transferred to the stone. If you use cheap, uncoated paper that isn't an issue.

I prefer loose SiC grains for stone flattening FWIW, but there's nothing magical about sandpaper, either good or bad. It works as well as anything else provided you understand what you're using and select the right type.