Oh sorry I forgot the link:
Isn’t the issue du to the fact that Mautic uses only 1 mailbox for sending the emails. So postfix does its job properly because the sender is always the same. Mautic only adds some headers for the other users, but send using always the same mailbox. (I’m not 100% sure about what I’m suggesting since I’m quite new to Mautic but it’s my current understanding of how it works)
Please let me know if you find a way to make it work