OnePlus/Oppo Device Workarounds
Volume Not Changing If App is in Background:
OnePlus/Oppo has added a "security feature" to their phones to block all volume changes while an app is in the background. This happens on the OnePlus 12 at minimum, and it seems to have begun on devices as far back as Android 12.
Luckily, there is a workaround!
In Precise Volume, head to Settings -> Advanced -> Use background workaround.
I highly recommend letting it calibrate.
PS: This workaround might not work forever. If you believe you should have the ability to change volume in the background on a device you paid good money for, then please email OnePlus.
Volume Button Override showing on top of device's built-in volume interface:
To fix this, go to your phone's Settings app. Select "Sounds & vibration." Now, disable App-specific volume. Unfortunately, this is the only way to stop the default UI from showing.
2025 Update Information
OnePlus keeps changing things about their audio system. You may need to disable "Device Volume Sync" for your individual Bluetooth devices. Go to your OnePlus device's Settings app -> Bluetooth -> (Your bluetooth device) -> Device volume sync.