Jay Gairson writes:
Steve: Fair enough. The issue ultimately was with Postfix, but I only encountered it because I misread the Mailman3 configuration wiki as setting "virtual_alias_maps = hash:/path-to-mailman/var/data/postfix_vmap" to be /path-to-mailman/var/data/postfix_lmtp.
Thanks for the followup!
For future reference for those who are following along, while that's technically a Postfix configuration issue, it's the kind of thing that people like Mark and Abhilash are at least as likely to notice as most of the folks on Postfix lists. Bottom line: don't hesitate to report these issues to us, but if it could be the MTA, report to their users' list too. More eyes, etc.