[Koha] overdue notice lists all checked out books

Hi, I noticed this after upgrading to 3.18.1 (from 3.16). If a patron has overdue items, they receive the overdue notice, but the list of overdue items includes all the books they have checked out, not only the overdue ones. I also get this error on the cron run, not sure if it's related: Use of uninitialized value $borrowernumber in string eq at /usr/share/koha/bin/cronjobs/overdue_notices.pl line 544. Installed on Debian from the koha repositories.