This happens to me when I drag a midi file into a song from studio one's file browser. Sometimes, doing it this way I get a dialog box asking if I want to load default sounds, sometimes it doesn't bother to ask and loads the default sounds. To always get the dialog box, I've found that by using Windows file explorer and right clicking the file and opening it in studio one will always trigger the dialog box so I can click "no" to loading default sounds. Then I can choose my own instruments one track at a time.