I must be really close on this, and I hope it's ok that I'm jumping in on this thread. I am also using the mailman-full package on Ubuntu 18.04 (and have been successfully running a bunch of lists now for almost 2 years!). I think I'm doing what the docs say to do, in order to override specific list footers, but to no avail. Everything else about my system is working great - I just can't customize templates.
So I have a template file here: /var/lib/mailman3/templates/lists/f2p2-test.udfcd-lists.org/en/list:member:regular:footer.txt
I had to create everything below templates/ as that folder was empty on install. I made sure what I created had the same permissions as the templates folder. I'm not sure what else to do. Is there a configuration file somewhere perhaps the identifies the path of the templates folder and maybe I need to adjust it?
I appreciate the help.