I am a private pilot. Flying my unpressurized Cessna I never had to fly over 10000 feet but my gps always gave me a fix up to at least 7000 feet. iPad GPS is not a real gps. Real gps use satellite signal to triangulate their position. iPad GPS used cell towers to triangulate their positions. To the maximum height is more a maximum distance from ground cell towers which at 10000 feet is roughly 3 kilometers.
The GPS module in the Ipad is a "real" GPS, albiet it has a small antenna.
It uses the cell phone towers to help get a triangulation faster and that is known a assisted GPS (A-GPS).
I have had my Ipad in use at 41,000 and it was able to give me a usable signal but I had to go back in the main cabin and sit by a window to make it work. I dont think I was able to get below 100m of accuracy with just the internal GPS.
Forward windows on airliners (L1,L2 and sometimes L3) are too thick for descent signal penetration even with an external antenna.