Not sure this will work for everybody, but I ran into this today and someone on the team suggested “have you tried Normal“?
Turns out he was right. The following seems to nicely restore your window:
if (myWindow.WindowState == WindowState.Minimized)
myWindow.WindowState = WindowState.Normal;
That works just fine, restoring the window to Maximized if needed. It seems critical to check for the minimized state first as calling WindowState.Normal a second time will “restore” your window to its non-maximized state.