Hi,
It seems an inconsistency. Please let us know the selection options you have chosen. Also check whether the material type has been changed or the quantity update for the material type has been changed in between.
In addition, refer the note: 1772782 - MB5B: Detailed information about opening and closing stock
which explains the system design and the logic for MB5B. Refer the same and revert back.
Regards,
AKPT