This is not a mystery:
a REPLY ALL by the recipient would no doubt go back to the list regardless of your list definition parameter that you have shown (unless recipient subscriber was NOPOST (or perhaps REVIEW)).
Now it may be that the recipient's MTA (defaults) implicitly to a
REPLY ALL as opposed to a REPLY.
[log in to unmask]" type="cite">The list was set as Send= Private So all subscribers could post to it. However, all responses were to be directed to the Sender, not the List Reply-To= Sender,Ignore *But* the reply did go to the list. I am still trying to get the original mail, and it is possible the response was made on a mobile device, which may not be too standards-compliant. Members of the list themselves may not have recognized what had happened, however, the list owner and her boss sure noticed it.