Seeker FWIW - I don't think it's the electronics, rather the microphone
This is interesting because I was really thinking it was related to (or caused by) some kind of software setting (in the phone/OS) that prioritizes voice over other sounds. But if adding an external mic makes such a difference, maybe it’s just the microphone that prioritizes voice.
Re that Zoom phone mic, I have looked at that before (I mean, online), and at this point, I don’t remember why but I decided not to buy it. Those models are all several years old at this point. I notice they all use lightning connectors, which is fine for me because my iPhone is old but eventually I’ll get a new phone and it will be a C-port. I wonder if Zoom has a newer model or if there are other brands that are good for piano recording….