Windows 8.1 Audacity 2.3.1
I have noticed my stereo files have been converted to mono after export. I have retained the original files.
If you are exporting as MP3, there is an option to force the export to mono (see: https://manual.audacityteam.org/man/mp3_export_options.html)
To export a stereo MP3, that option must not be selected (it’s off by default).
For other formats, the problem must be something else.