I make an app full-screen on my Mac Mini by pressing the green expand button in the top-left corner of the screen.
This results in the automatic creation of a new "Space" for this app. When I am in Mission Control, I now have two spaces: the Desktop and the full-screen app.
The Mac has one display connected to it. When I turn off this display (by pressing the display's power button), and then immediately turn the display back on, the full-screen app is still on screen. In other words, turning off and on the display does not change anything (which is what I desire).
The problem is that if I turn the display off, wait awhile (around an hour), and then turn the display back on, I am now looking at the Desktop Space. In other words, the active Space automatically changes to the Desktop Space when the display is off for a certain amount of time.
What's strange is that this issue never used to occur until recently (so this might be a High Sierra issue). Also, there is no app open in the Desktop Space that is "pulling focus" away from the full-screen app; I just see my Desktop wallpaper. Also, this happens no matter which application is made full-screen.
Toggling the following options in System Preferences → Mission Control does not solve the issue:
Automatically rearrange Spaces based on most recent use
Displays have separate Spaces
Does anyone know how I can ensure that the active Space will not change to the Desktop space, when the display is turned off for awhile?
macOS High Sierra, version 10.13.5.