Apple restricts this as part of the location services permission though, and to clarify, it’s always happened very shortly after touchdown, just as we started to taxi, where probably the terminal WiFi is not in range.

After thinking about it more, I think they somehow know what flight I’m on and are using flightaware or a similar service to know when the plane touches down.

The question then is how they know I’m on the flight. They can do this if you share your calendar with them, but I don’t. The only thing I can think of is either the airline is selling them the data or they’re getting it from my credit card company. Airline flights are unusual on credit cards in that the entire itinerary is on your credit card statement, not just “American Airlines” or whatever.