There may have been changes made after the sales items are transferred to MC but these group changes don’t appear on MC. Therefore, you need to edit the sales item groups in MC too.
For this, go to the Cost Control - Sales Items menu and click Open File. On the search screen that opens, write the name of the stock item with the incorrect group to the receipt area and click OK to search.
Mark the stock item and click the Change button. On the screen that opens, mark the New Sales Item Group and select the correct group on the right side. Click OK, then apply and save.