samba - Sep 2018 - 4.8.5 + TimeMachine = Disk identity changed on every connect, cannot backup

I configured Samba 4.8.5 on Debian with vfs_fruit as a TimeMachine destination
and while it detects it and does the initial backup to some extent (300GB out of
600GB), TimeMachine then fails with a message about the disk identity having
changed.
Options are “don’t backup” and “backup anyway”. When using “backup anyway”, the
backup creates a secondary sparse image and starts from scratch, and won’t even
touch the existing sparse image.

While I understand that this is specific to macOS, TimeMachine and vfs_fruit
combined, the Samba mailing list is the only one I could find regarding Samba
and vfs_fruit.

Is there anyone experiencing this issue, and is there a way to get more
information from either macOS or Samba to find out why TimeMachine thinks the
disk identity keeps changing?

Underlying FS for the share is ZFS, but I tried it with EXT4 and that gives the
same result.

John