Hi J J,
Hope I understood your issue but quality stock is just a stock type and it's considered as a available stock for planing purpose. So even though you post stocks to quality, SAP will consider it in net planning.
If you want to exclude it from MRP then possibly you need look for some other alternative.
e.g. define a separate stor loc just for quality stocks & exclude it from planning and once quality check is performed, move it to relevant other location.
Hope this helps.
Thanks.
Kind Regards,
Anup