If you're talking about the floating window that you move over to your second screen and hit full screen to display the karaoke lyrics, etc on the second screen, I'm having the same issue. It comes up when loading and then disappears, when I click on the window in the council that should bring it up, nothing happens.
Thanks for your comment, unfortunately mine doesn't respond to trying to click on it at all, it just disappears either way and doesn't respond in any way to clicking on it. Do you have to click on it each time you open Dex 3 now or after doing it once did that solve it from then on? Just curious.