[Zubersoft]

I definitely have no idea how to reproduce what you've described. I tried selecting a song in my library, tapping Share->Export Files, selected the Downloads folder in my quick access list as the destination, tapped OK to accept the settings, and the file was output successfully. I then checked the storage location and no files were impacted by the export. In general, no non-temporary files should be modified or delete by exporting - there is no code in place that would do that. Some files may be placed in the TempState directory if they are going to be uploaded to a cloud service, and then they are deleted after the upload is successful. 

So let me ask this - what was the location you selected in your quick access list? Is this with the Microsoft Store or FastSpring version of MobileSheets? If you backup your library (Settings->Backup and Restore->Backup Library), uninstall, reinstall, and test again, does the same issue occur?

Thanks,
Mike