Thank you, yes, I've read all the documentation. My system works properly (transport_maps, local_recipient_maps, relay_domains), I don't think it's an unusual configuration at all. Mail and all functions for configured lists work properly (I have > 100 lists on this server), I just have the same issue as the OP -- mail to <address-does-not-exist@lists.my-domain.tld> is bounced with the message "mail for lists.my-domain.tld loops back to myself". One common example is <list-used-to-exist-bounces@lists.my-domain.tld>.
Postfix relay_domains points to regexp: /path/to/mailman/postfix_domains, and postfix_domains contains the line /^lists\.my-domain\.tld$/ lists.my-domain.tld (and mail for legitimate lists is routed correctly via lmtp), so I'm not sure what's not right.
Grateful for any insight you can offer! Thanks!