RHEV: disk fails to extend but results in huge disk capacity

Issue

Attempted to extend a thin provisioned disk attached to a Windows VM from 35GB to 50GB.
Following is seen in RHEV-M Events tab:

Failed to update VM 'XXXX' with the new volume size. VM restart is recommended. 
Size of the disk 'XXXX' was successfully updated to 17923 GB by 'XXXX.' 

Logs from VDSM for disk extension show it actually aborted before doing any extension because requested new size (50GB) was already smaller the current size (18TB)

__init__::481::jsonrpc.JsonRpcServer::(_serveRequest) Calling 'VM.diskSizeExtend' in bridge with {u'driveSpecs': {u'domainID': u'c8d3798c-3e56-4c5e-a0c2-436488c9f393', u'volumeID': u'bf3ff80d-3908-435c-a624-b6a0ca079970', u'poolID': u'00000002-0002-0002-0002-0000000000f6', u'imageID': u'b4be957e-4b5f-4e3a-bbfd-5d9e281603c7'}, u'vmID': u'282a67de-9c97-4c2f-b8ea-d484de6185bb', u'newSize': u'53687091200'}

utils::739::root::(execCmd) /usr/bin/qemu-img info -f qcow2 /rhev/data-center/00000002-0002-0002-0002-0000000000f6/c8d3798c-3e56-4c5e-a0c2-436488c9f393/images/b4be957e-4b5f-4e3a-bbfd-5d9e281603c7/bf3ff80d-3908-435c-a624-b6a0ca079970 (cwd None)

utils::759::root::(execCmd) SUCCESS: <err> = ''; <rc> = 0

vm::4703::vm.Vm::(_diskSizeExtendCow) vmId=`282a67de-9c97-4c2f-b8ea-d484de6185bb`::Requested extension size 53687091200 for disk vda is smaller than the current size 19245681344512