I have discovered there is a bug in the latest pi-bluetooth package that prevents the use of the internal BT device when external BT devices are plugged in. V 0.1.13 is the last working version (it seems) The following fixes it.
https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=282948&p=1812375#p1743444
I realise this is probably only for anyone still running Raspbian and possibly RaspberryOS (which is unsupported) but my find may help someone else - it was a little obscure to find. This may affect HomeAssistantOS as well if it uses this package.