Current behavior:

1. Have cross-wiki notifications and local wiki notifications.
2. Open the Notification panel and click on 'Mark all as read'
3. All local notifications become grey indicating Read status; the cross-wiki notifications disappear from Notification panel indicating their Read status.
4. To verify that those cross-wiki notifications are, in fact, were read, go to the wikis from which they were from - the notifications would be marked correctly as read there.