Hey OneFisheye,
Since it's still opening on that secondary display with that option enabled, then it appears that GarageBand has been assigned that space in Mission control. I suggest that with GarageBand open, you then open Mission Control, and drag the GarageBand application window to your main space.
Use any of these methods to open Mission Control:
- Move a window to a desktop space: Drag the window to that space in the Spaces bar.
- Move a window to a full-screen space (Split View): If an app supports Split View, you can drag its window to a full-screen space in the Spaces bar. The combined space shows the names of both apps. Click the space to see both apps in Split View.
If your windows are grouped by application, an app icon appears at the bottom of each group of application windows. Drag the icon to move all of the app's windows at the same time. To turn on this feature in OS X El Capitan, choose Apple menu > System Preferences, click Mission Control, then select “Group windows by application.”
Use Mission Control on your Mac
Take care.