When exporting (multiple tracks) you can choose between Interleaved, Mono-Downmix, etc. When “Interleaved” is selected, all files will be exported as stereo, no matter if they are mono. Its the other way round, when “Mono-Downmix” is selected, then all files are exported as mono.
It would be great if there would be an additional “standard” mode to export the tracks as they are.
If the track is a mono track, export it as mono, if it is stereo, export it as stereo.
Right now, you cannot do both exports simultaneously. You need a separate export for each.