The so-called "feature" referred to as "Full Screen" is ported from iOS, the land of pygmy screens. It does not support reasonable sized or multiple screens, only stretches the window on the MAIN screen to eliminate the MenuBar area, then darkens the secondary displays. Avoid it at all costs.
You can drag any window onto the second display, or span multiple displays, and manually stretch it to whatever size you like manually. Just don't think "Full Screen" was ever intended for you.
This isn't a notebook forum, but I wonder if you make your 27" to be t he main display it will? of course if that screen is not present...