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 subtrack 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.