During a proof of concept deployment at Veeam University Hospital, it is noted that only...

To make backup files immutable in a Scale-out Backup Repository, you need to use XFS extents with the reflink attribute enabled. This attribute allows Veeam Backup & Replication to create immutable backup files using the fast clone technology. If some of the XFS extents do not have this attribute enabled, they will not support immutability and the backup files stored on them will not be protected from deletion or modification1