exported snaphot integrity

[heh74]

Hello folks,
I'm using QNAP iscsi targets to backup VMs and I'm replicating snapshots to secondary QNAP (both TS-873, latest firmware).
I'm able to export snapshots from secondary QNAP to external USB HDD.
I'm able to connect these .img files(QCOW2) from external USB HDD via qemu (qemu-nbd --connect) on Linux (Rocky Linux) and sucessfully restore data.

BUT if I checked any of exported images via qemu-image check I was surprised. None of them were error free!
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Typical result:

Leaked cluster 1 refcount=1 reference=0
Leaked cluster 2 refcount=1 reference=0
Leaked cluster 262144 refcount=2 reference=1
ERROR cluster 262145 refcount=0 reference=1
Leaked cluster 524288 refcount=2 reference=1
ERROR cluster 524289 refcount=0 reference=1
Leaked cluster 786432 refcount=2 reference=1
ERROR cluster 786433 refcount=0 reference=1
ERROR OFLAG_COPIED data cluster: l2_entry=8000002000000000 refcount=2
ERROR OFLAG_COPIED data cluster: l2_entry=8000004000000000 refcount=2
ERROR OFLAG_COPIED data cluster: l2_entry=8000006000000000 refcount=2

6 errors were found on the image.
Data may be corrupted, or further writes to the image may corrupt it.

5 leaked clusters were found on the image.
This means waste of disk space, but no harm to data.
1024000/1024000 = 100.00% allocated, 0.00% fragmented, 0.00% compressed clusters
Image end offset: 536884019200
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Message "Data may be corrupted, or further writes to the image may corrupt it." is disturbing and definitely not OK.