snapshots from snap-sync used for incremental transfer

I have a volume that is mounted on `/mnt/forBackup`. It has subvolumes `@`, `@home`, and ` @myStuff`. I configured the time machine setup:

> timestamp_format long
> snapshot_preserve_min 18h
> snapshot_preserve 48h
>
> volume /mnt/forBackup
> snapshot_dir btrbk_snapshots
> subvolume @
> subvolume @home
> subvolume @myStuff

Then I wanted to backup the snapshots to an external USB drive, so I added configuration `/etc/btrbk/btrbk-backup.conf` with

> target_preserve_min no
> target_preserve 0d 10w *m
>
> volume /mnt/forBackup
> target /run/media/marko/Backup-2
> subvolume @
> snapshot_dir btrbk_snapshots
> snapshot_preserve_min all
> snapshot_create no
> subvolume @home
> snapshot_dir btrbk_snapshots
> snapshot_preserve_min all
> snapshot_create no
> subvolume @myStuff
> snapshot_dir btrbk_snapshots
> snapshot_preserve_min all
> snapshot_create no

When I run `btrbk` with `-c /etc/btrbk/btrbk-backup.conf --progress` I see that only 15GB are transferred for `@`, 1.3GB for `@home`, and ~100KB for `@myStuff`. There is about 900GB in these subvolumes and I would expect the initial transfer to be about that size. Where is my mistake?

**Update**: It seems that `btrbk` was incrementing a snapshot that was stored on that same external disk using `snap-sync`.

Start with the two configurations, /etc/btrbk/btrbk-backup.conf and the time machine setup, and reproduce the command `btrbk -c /etc/btrbk/btrbk-backup.conf --progress`. Trace which snapshot source is selected, including the snap-sync snapshot on the external disk. Done means the cause of the unexpectedly small transfer is documented and the initial transfer uses the intended source.

Written by the indexing model from the issue text.