Reaccom doesn't appear to kick in the second you fall below MCT, which is just as well for various reasons. So there is some wriggle room left in case you get a fast run over the jetstream. But at the point it becomes impossible then the App should refresh, your old BUD booking will still sit in MMB, but the App should also show the new flight. It's sometimes easier to go to the list of all bookings to make sense of it.