If all of the tracks are mono (and centre panned) the exported file will be mono.
To force the exported file to be 2 channels (even if the audio is actually mono), add an empty stereo track (Tracks menu > Add new > Stereo track).
I’m not sure why you would want to export a mono recording as a stereo file, but that’s how to do it.
I indeed on this recording have all the tracks centre panned. The solution came to me as I was driving to the supermarket but my wife wouldn’t let me put off the monthly shopping to come back and try it.
Now the shopping is done and I’ve got everything working ok again.