Multiple email accounts in one inbox

After upgrading to Big Sur my mail configuration has changed. I have multiple email accounts and I used to having a consolidated inbox named "All Inboxes". This is gone after the upgrade to Big Sur.

I have searched all the menus, preferences etc. but have not been able to find the pixie dust required. When I click "favourites - add" the item "All inboxes" is greyed out.

Any idea how to display "All inboxes"?

Best regards