Comment 1 for bug 1734285

Files in /etc/apt/preferences.d/ are read when unattended-upgrade is run and not when u-u.service is started thus it is safe to assume that configuration changes take place without restarting u-u.service.
On the other hand there are cases when u-u does not respect pinning like the one reported here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=838920
Are all the repositories trusted on the affected systems?