Mono + Mic instead of stereo sound for bluetooth headphones on ubuntu

Short answer : There is no solution as long as you are using Linux and a Bluetooth headset.

Long answer: For music playback only A2DP is suitable, but it does not have an INPUT mode so use of the headset for VoiP isn't possible. The HSP/HFP mode uses the CVSD codec which is sampled at 8 kHz, same as used in telephony systems (so sounds like an old telephone, if at all).

A fix was introduced in pulseaudio 10 for automatic switching between A2DP and HSP/HFP profiles, based on detecting that some phone application wants to access the microphone or not. But this doesn't work for all headsets and applications.

You may read the long-running bug-report Bug #508522 : Add automatic switching to HSP/HFP from A2DP when a mic is needed, starting in 2010 (!), and whose status today is still "New" after having been closed and re-opened during its long history.

Such headsets are reported as working perfectly well in Windows and MacOS, so the problem is only with the Linux pulseaudio implementation of Bluetooth. You would need to use a USB headset to avoid the problem.