According to this documentation:

https://backports.debian.org/Instructions/

Backports should have a priority of 100 in the Release file:

    " /All backports are deactivated by default (i.e. the packages are
    pinned to 100 by using ButAutomaticUpgrades: yes in the Release files"/

This seems not to be the case with Devuan. I had to pin it explicitely
in the preferences file.

Regards

Jochen