Hi Jon,
I encountered the same problem while sudo apt upgrade but after this command: sudo ss -lp "sport = :domain" the output was two line long and in process column I had “systemd-resolved”.
I used this commands to fix it:
sudo systemctl disable systemd-resolved.service
sudo systemctl stop systemd-resolved.service
sudo systemctl restart dnsmasq.service
You can also change port number in /etc/dnsmasq.conf if you don’t want to disable systemd-resolved.
Best regards
Karol Konkol
Husarion Team