As explained in https://community.metabrainz.org/t/itunes-change-alert/429010, Apple recently split the domains for Apple Music and iTunes. Our current URL cleanup régime only accepts iTunes-formatted URLs, so any link submitted with the domain "music.apple.com" needs to be swapped with "itunes.apple.com" before it's added to MB.
UPDATE 5/24: Even if a user follows this protocol to add a link, if they click on it the link will revert to the "music" domain when loading the page. It might be a good idea to allow links to "music.apple.com" and/or clean up existing links. (We also might need to create a standalone "Apple Music/iTunes" URL relationship type, but that's a separate ticket.)
UPDATE 11/3: Apple Music URLs now use a new icon (the pink and blue eighth note on a white background logo). When and if this ticket ever gets implemented, we should add this icon to links in the sidebar.