I opened a second Viewport and dragged it to my second monitor. This way I can see my game on fullscreen while working on materials or blueprints on the other screen. I think the problem exists also if you use just one viewport and put it on the second monitor.
Of course, I want to have my Viewport 2 always open on monitor 2, regardless of what Editors I have open on monitor 1. However the moment you open a tab on monitor 1 that’s not the main one (Blueprint editor or whatever) Viewport 2 disappears and nothing is shown on the monitor 2. In order to see it again you need to activate the main tab, so that the Blueprint Editor is not visible any more. So if I want to see my Viewport, I need to hide any Editor I am actively working in.
I don’t see how this is useful. I hope you share my opinion and consider adding an option for changing this behavior.
P.S. I know there is a workaround - by opening editors in windows instead of tabs. However, this is not optimal for me, as it takes much more effort to switch between them. And they also clutter my windows taskbar.