Now that was weird... The "This Site" contextual scope was showing in the Scopes Dropdown, but the "This List" scope was not showing at all on any list:

 

 

I checked and changed the DropDownMode property to see if that would bring the "This List" scope back ...but no.

 

Call it "years of trial and error" experience to find the solution for this problem: turns out that accessing the site with the "Custom" zone (configured in AAM as "http://localhost") was causing it ...when accessing the site with the "Default" zone (configured as http://2k8x64Moss) the "This List" contextual scope shows just fine ...I'd call that a bug, MS probably calls it a feature ;-)