Can't add an external domain for sharing site

Hi

In sharepoint Online we have limited external sharing by domain on organizational level.

Here I can add external domains to the approved list.

Additional to that we also have limited external sharing by domain for a site.

Bur here I can't add any new domains. Can't see why since I'm a global admin

I can remove one of the existing approved domains but I cant add any new ones. 

Why is that?