This is an in-built protection mechanism used by Windows. Please refer to the following link for more information.
Permanent resource exhaustion setting.
Permanent resource exhaustion represents a hard limit on the LUN’s capacity consumption. This prevents the LUN from unexpectedly using capacity when other LUNs need it. If resource exhaustion is permanent, the storage array cannot allocate more resources to the LUN. The Windows partition manager sets the LUN to offline when the permanent resource exhaustion threshold limit is reached.
If you encounter this situation, please expand the storage space on your QNAP NAS or delete unnecessary files to reclaim space for the iSCSI LUN.