I have had this problem on and off on my iPhone 12 Pro Max, and both Air Pods Pro, and Pro version 2. Playback will switch to music on the watch for no reason, and the only way I can get it to switch back is to power cycle the phone. Important to note that the watch is on the charger when this occurs.
This has gone on for months now, and a new problem has added to the mix. The Airpods will periodically pause playback for no apparent reason. In hours spent trying to resolve these problems, Apple even ended up replacing my phone with a new one a few weeks ago. Even after this, I still encountered the problem with the new phone, I purchased new Airpods Pro V2s, and nope, still does it at random times.