Diligent problem solving . :) Congrats.
If you can't solve the issue by lapping with the Atoma, the stone is faulty. If you read some old posts you will see the 'graininess' problem has occurred with other stones also. Notably the Norton 4/8 & also my 12k Shap Pro. It is usually just the top skin that needs to go but at least you have notified your supplier in case you need to claim warranty.