It seems they now have some sort of logic to determine when one is crossing a MM threshold. When I crossed 2MM last year I got a "Your 2 Million Miler day is here" notification in the app a few hours before departure of the first segment in the itinerary (not the one that actually put me over the threshold). Not sure if there was automation in some system to determine that, or if there was a human "behind the curtain" somehow sending a notification based on data they had.