If my upgrade had been confirmed in the App, I would definitely have escalated to the FA and showed my boarding pass to them and simply said, there is someone sitting in my seat. I assume their app would have showed you in the seat. If after that, you get commanded to return to your original seat, then of course you comply.

That is one of those rare cases of actual upgrade shenanigans. A stand-by (was it a non-rev?) just being given the first class seat. Needs retraining for sure.