Hi,
It seems to be an inconsistency. A similar situation is explained in the note: 1701875 - MSC2N: Stock inconsistency after status change. You may check whether your situation is similar to the case. Also check whether any own development is causing the inconsistency.
Additionally, refer the KBA: 1618888 - How to deal with stock inconsistency? and the note: 520010 - FAQ: Inconsistencies in inventory management. If issue is still not resolved, I would suggest to raise an OSS ticket to SAP.
Regards,
AKPT