Bluetooth headset only plays sound in mono

Bluetooth devices need to support so-called profiles, which basically describe what the device can do. If, for example, you want to use a Bluetooth keyboard, it needs to be compatible with the Human Interface Device or HID profile.

For headsets and other audio devices, there are multiple relevant profiles: The Headset Profile (HSP) and/or the Hands Free Profile (HFP) are used when you want to use the device in two directions, so to speak - speaker output and microphone input. With HSP or HFP, the available bandwidth gets split between the speaker and the microphone, so the quality of the audio output will be lower. Maybe that's what you mean by "mono".

If on the other hand you use the Advanced Audio Distribution Profile (A2DP), the whole bandwidth is used for the audio output. Because of that, the quality of the audio output will be better as with HSP/HFP, but you won't be able to use the microphone.

You'll need to look into which profiles are supported by your specific device. A device can support multiple profiles, but only one at a time. For example, my headset supports HSP/HFP as well as A2DP. You can select which profile you want to use in the audio settings, but you can only use one profile at a time.

You also might want to look into which other Bluetooth devices are connected to your machine. Typically, you can only use one device per profile at a time. So if you are, for example, using a Bluetooth speaker with A2DP, you can't use Bluetooth headphones with AD2P at the same time.

By accident I found that if I remove the headphones from the Bluetooth device list before shutting down the computer and then add them again after it is turned on it gives me both profiles and I can select the stereo profile and it works fine.

If I don't remove the headphones before shutting down then I have to remove them and then reboot. Not the best solution but it does work.