There is a known issue whereby an Exchange Online user is not getting added to the authorised senders list in the DL. It allows the user to add, then click Update however upon re-entering the DL, the users that were previously added do not appear therefore adding them has failed. Sometimes no message appears, whereas other times there have been occurrences of a success message appearing saying that the user has been added. The success message may appear or may not appear.
Latest Update and Guidance
This issue is being investigated under PRB0165916. Users facing this issue need to reach out to the email@example.com .