OK, now stop Gig Performer and open a program on your internal Display and minimize it, what happens with your external Display?
Found something interesting:
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Why does my Mac black out the second monitor when I fullscreen?
In System Preferences, find and click on “Mission Control”. Look for the option that says “Displays have separate spaces”. Ensure that this box is checked. This setting allows each connected display to act as a separate space, which is essential for using apps in full-screen mode across multiple monitors.