When migrating the wikimediacz-l mailing list archives into Mailman3, they were briefly public (for a minute or two) during the import process before the private setting kicked in.
<Majavah> I was able to see the list of threads including some summaries of message contents, didnt't test viewing individual threads
I (Legoktm) think we need to run the sync_with_mailman step so hyperkitty knows the list is private *before* we import the archives, right now it only happens afterwards.
Upstream: https://gitlab.com/mailman/hyperkitty/-/issues/380