What is the need to backup CIFS via UNC ? Filesystem is already presented from NAS itself (via CIFS/SMB) so why not back it up from there itself. I believe Rubrik does SnapDiff with NAS side snapshots (this ensures data is consistent and no files are skipped)
I don't know why flexclone is created/needed to backup CIFS shares, it is just another common "snapshot" shared with its parent. Instead just back it up from parent volume snapshot.
I think there was another thread raised earlier on the same topic.