Alex Pankratov :
Jul 24, 2019
Generally speaking a file is picked up for backup if the timestamps are different between its source and backup copies.
Enabling shadow copying causes VMs to be automatically flushed to disk in a consistent state every time a volume snapshot is created. This also touches the last-modified timestamp, triggering a backup. If you are not seeing this, then I'd guess the root cause lies with luks.
You can work around this by forcing backup of (seemingly) unchanged files using this option - https://bvckup2.com/support/forum/topic/415