The Stark phone (Arkenstone) shows an engineering menu or strange screens instead of the app — why, and what to do?

If the volume up button on the Arkenstone is held down at power-on, the phone boots into the engineering menu.

This happens when a button stays pressed after a hit to the right side of the phone (e.g. the upper volume button).

What helped in the chat: restart the phone. One owner first got garbage on screen, then heard three sounds, and after a restart everything was back to normal.