So what you want to backup is a btrfs unmounted subvolume (/home/data/dossiers) compared to main mount point /home.
If you mount that subvolume, it will be backuped.
On my system /data is a subvolume of /
and I have it declared like this in my /etc/fstab.
/dev/mapper/cr_nvme0n1p2 /data btrfs subvol=@/data,nofail,noatime
If it is mounted it will be seen, and backup'ed.