This happens if you choose a track stack for export. To work around the issue, click the disclosure triangle to open the track stack and select the sub-track within the track stack you would like to export. Repeat for any additional sub-tracks within the track stack that you want to export.
You can also choose "Export All Tracks as Audio Files". In this case, each sub-track within the stack and all other tracks in the song will be exported.
If you want to create a stem file from a track stack, select any or all folder regions in the track and use the "Bounce Regions in Place" command.