“This site is in maintenance mode” for live site login

When logging into a site I’m seeing the “This site is in maintenance mode” notification when logging into a site that is live and site visibility is set to “Allow search engines to index this site”.
The site is part of a Multisite network and some sites are set to “Visible only to administrators of the site”.
Screenshot: https://www.evernote.com/shard/s31/sh/22afc634-3965-4198-85ea-9f718e989015/be6ab67c72bb2ee041309857fba3fcba

I have the visibility set on a site-by-site basis but some site admins cannot log in to edit their content. This is not happening for all users.