Very interestng! Upon restarting the system (down momentarily for taking a snapshot) and starting Mailman again, all of these missing [List] messages emerged from their timewarp and were delivered to the members / subscribers. Just before shutting down the system I discovered that the apache2 webserver had been off the air due to a module not being enabled yet. As of yet, I was unaware of any requirement for a web server to be online (beyond possibly gunicorn) in order for Mailman to run its lists. Does that speculation have a factual basis? Or do I risk starting a vicious rumor amongst the impatient readers who are not still here once we deduce this pair of observations were merely correlated in time, with no cause-and-effect relationship? Before this situation healed itself, the thought had crossed my mind to peek and see if anything was stuck in the queues: https://docs.mailman3.org/projects/mailman/en/latest/src/mailman/rest/docs/q... While that could have provided peace of mind that email messages were merely delayed instead of vanished, even in hindsight I do not see how that would have led to repairing the situation.
In the absence of a hypothetical command: mailman status --verbose about which diagnostic tools or clues should I learn?