This is the case for users that never had notifications in a wiki to which they are logged in (found when checking the fix for T142148: Special:Notifications fails to show foreign notifications (or any UI at all) on wikis where you've never received a notification.
Note: if there are read notifications, the issue is not present.
- Log in as a user that never had notifications from the wiki to which a user is logged in.
- Expand the flyout and click on 'All notifications'. While Special:Notifications is loading, 'no-js 'You have no notifications' is displayed.