Powerbeats Pro's Volume Control Not Working After iOS 13.1.1 Update.

Exactly the same issue here - definitely a bug in IOS 13 (I’m on the latest patch and the issue continues).

Steps to resolve have to be carried out each time I want to use the Powerbeats Pro.

1. Access Bluetooth in settings.
2. Select (i) icon next to device name and select ‘Disconnect device’ if not already disconnected.
3. Select ‘forget device’.
4. Put both earphones in the Powerbeats Pro charger case and hold down the ‘Reset’ button for approximately 15 seconds (until the light on the front flashes red and white simultaneously.
5. You should now see the ‘Connect’ pop up display on your iPhone (as if connecting the earphones from new).
6. Follow the connection steps - and all should be ok (until the next time you want to use them).

I selected ‘No’ to Siri and Announcing Messages.

I suspect the code for earphones has been altered to account for AirPod Pro - and something has gone amiss for Powerbeats Pro in the process.