Can high altitude cause any issues with seals?

Good question!

Air pressure is a tough one with any device. Most are not so sealed up that tight they can’t equalize and the volume air inside is not that much so it doesn’t take much to balance out.

The quantity of pressure difference and the speed of change is the bigger risk. As in example you have a iPhone in the ISS and the location it was in had a sudden decompression to zero the weakest point for the air to escape from will give way first. But that is the most extreme case you could encounter. And we already know NASA has iPads in use on the station so I’m sure they tested things out to make sure things are safe. How NASA chooses tech for the International Space Station, and why AI could help get us to Mars

Other than Apple I don’t think anyone could answer you with a definitive answer.

I can tell you I’ve flown often enough that my iPhone and iPad have not encountered any issues. Thats in both unpressurized and pressurized planes.