When I use a RadSplashScreen at the launch of my application, the application's main window doesn't receive focus and appears behind other windows. I've seen a reference to this type of behavior on another thread regarding RadWindow (https://www.telerik.com/forums/mainwindow-losts-focus-after-child-radwindow-is-closed), but the solution there doesn't seem applicable to this control.
I created a sample application (https://github.com/3milio/RadSplashScreenHidingMainWindow) with VS 2017 following your getting started guide. Mind trying it out and seeing if I'm missing a step? For me, when I hit F5 in the debugger to run it the splash screen appears on top of it but then when the main window gets instantiated it hides behind other programs.
Also, when I first cloned the repo and ran it in the debugger it didn't demonstrate this behavior. But any time after that, it did. That includes running it with the Release configuration, as well as simply running the .exe from Windows Explorer.