I believe it is a bug, as sometimes I get similar behaviour when I open an app, and I then start screen swiping before the app has finished opening. Or when I start up (I use the auto-resume features heavily), and occasionally the order the app's restore in results in the windows of an app not running in fullscreen mode, opening up on a desktop belonging to an app in fullscreen mode.

I would say it's absolutely a bug - I have also seen several bugs with Fullscreen mode in general. As nice as it would be to use FS without losing my second monitor, I have not tried and would never use this hint. I don't want to start relying on functionality that I'm almost sure Apple will 'fix' in the next update or two. So, like the guy with the car metaphors, I say if you want to see multiple apps, don't use FS.