When I try to add an external link to a Facebook URL on beta.musicbrainz.org I get this error:
'This URL is not allowed for the selected link type, or is incorrectly formatted.'

Examples:
Adding the url 'https://www.facebook.com/BLAMETHROWERNZ' to:
https://beta.musicbrainz.org/artist/37ed57ca-2b52-4437-8e9d-0442f1d7c9e1

Adding 'https://www.facebook.com' works, adding any characters after that will throw up the error.
Editing an existing link, eg adding any text to the end of the FB relationships for this group: https://beta.musicbrainz.org/artist/2cabc728-c597-4a01-b15b-faeda2d41187
Also throws up the error.

I can add FB relationships to events fine.

Windows 10, Chrome, some scripts (will add more info/test more if needed)