Are you aware of the changes made to the Addresses panel?
Each mailbox now has an e-mail address associated with it, lets call it a mailbox address. If you want other addresses to deliver to the same mailbox, you set them up to forward to the mailbox address.
So its possible that the changes you are talking about occurred due to this change. DH had to choose an address that you had setup to be the mailbox address for each mailbox as well as update the other addresses as necessary so they “forward” to the mailbox address (instead of “deliver to mailbox” as it was in the past)