iPhone 8 volume changes by itself
Have reset, turned off sound check, made sure the case isn't doing it, set ringers and alerts high, tried equalizer off and on late night. Turned off Phone Noise Cancellation, tried Hearing devices on and off, shut off all other apps and bluetooth. Made sure iWatch was turned all the way up. Still while playing music in my car either with bluetooth or Apple Car play the volume goes down by itself to about halfway. Happens while on its own speaker and while using plug in or bluetooth headphones too. I shut the Facebook volume off too. I have the latest updates. Have googled until I am nuts.
iPhone 8 Plus, iOS 11.2.6