This is just a function of the software. The volume for headphones, Bluetooth devices, Siri, and onboard speakers all have different volume memories. The volume will always default to whatever it was the last time you used the device. e.g: Plug bluetooth speaker in and turn volume to 5, disconnect it from the phone and the main phone volume is at 3. When you turn up the phone (without the Bluetooth speaker) the phone’s volume will increase to 6, but after reconnecting the speaker it goes to 5 (the volume that speaker was last at) if you were to disconnect the speaker, it would go back up to 6.