Except there’s nothing that can be done support-wise if no one was watching. You can’t troubleshoot something no one saw. Over thousands upon thousands of flights, not once have I ever had a flight climb 14k feet past the altitude setting. In order to troubleshoot something like that, there’d need to be some input from someone that watched it happen, preferably with screenshots or something.
I would suggest always sticking with your flight at least until you reach your cruising altitude. That will always prevent the tape sneaking down unnoticed and things like that. But as far as I’m aware, through live flights and extensive testing, I can’t remember anyone ever noting that the plane climbed almost 15000 feet past the set altitude. If it was a bug, it certainly would be reported more than once.
I have accidentally tapped on the ALT button and left V/S as is, which would explain what happened here. That can happen. But that isn’t a bug, just me fat-fingering a button. And setting your device down to go eat is the perfect opportunity for your thumb to accidentally hit that button. I know that’s when I usually accidentally hit something. That’s a much more plausible explanation than an altitude bug that isn’t being massively reported.