iPhone 13 volume keeps turning down on it's own

The ringer volume on my iPhone 13 mini (OS 15.2) that is normally on maximum keeps adjusting down to 20% or so by itself. It can (and often does) happen while it's on the nightstand. Or in my pocket. Or on my desk. I’ve turned off the “change with buttons” in Sounds & Haptics, but doesn’t seem to do anything. I can still change the volume with the phone's buttons.

The volume buttons don’t work with my Bluetooth headset, so that’s not it. (They do on the phone, but not on the headset when the “change with buttons” is off.)