If your phone has saved enough of the map data in it's memory then it will track you based on that.
This has happened to me, although with Google, not FR24. I have had my phone on the meal tray, by the window, flight mode on, but location activated afterwards and my whole flight path was tracked pretty much exactly. Short haul flight mind you. When I went to my Google timeline it showed pretty much every turn we made and city we flew over, with a few blips along the way.