Solution found (at least for Android devices): Check this thread: Model S (2016) constantly turns down Bluetooth volume
Summary: Get a volume manager/locker app. The car keeps trying to adjust, but the volume locker will just turn it back. I can see it happen on my phone, but I don't hear any more changes.
Just my $.02 on this whole issue. The Model S/X bluetooth chip does not support the same newer BT protocols, but it appears it's trying to. I suspect that might be the issue. If you have developer options enabled on your phone you can see how it connects, but can't seem to force it another direction.
Summary: Get a volume manager/locker app. The car keeps trying to adjust, but the volume locker will just turn it back. I can see it happen on my phone, but I don't hear any more changes.
Just my $.02 on this whole issue. The Model S/X bluetooth chip does not support the same newer BT protocols, but it appears it's trying to. I suspect that might be the issue. If you have developer options enabled on your phone you can see how it connects, but can't seem to force it another direction.