From the moremember.pl page, if you try to add a patron to a list via "Add patron to list", it will silently fail if the patron does not have a cardnumber. This is particularly wild, because the patron_list_id and borrowernumber are sent in the request. But "patrons_to_add" is empty because there's no cardnumber. Koha::List::Patron::AddPatronsToList actually supports both borrowernumber and cardnumber, so it should actually be a fairly easy fix in theory. But we'd have to be careful about making the change to be clear across the board...