You only need viewports for this. Why are you using letterboxing for your minimap?
I think he means that he wants to apply letterboxing to his game, but it doesn't work for the HUD because it uses a view with a different size. So, when he resizes the window, his world view scales at a different rate than his HUD view, having the HUD all over the place.
Now, I can confirm that the position of the view is irrelevant, so it doesn't matter if it's at 0,0 or not (I just tested it), and it also doesn't matter if you are using views of different size than the window (because that's what letterboxing is for), but I can confirm that it doesn't work properly if you are using two views that are of different aspect ratio at the same time, because that's what the function uses for comparison to make the letterbox effect: the aspect ratio of the view vs. the aspect ratio of the window, without taking into account the other views. So, view1 may behave like having having black bars left and right, but view2 may behave like having black bars up and down, messing everything up.
So, a solution could simply be making all your views to have the same aspect ratio (not necessarily same size, mind you), but that may screw the size of your HUD, so you'll need to readjust accordingly
Also, keep in mind that if you are modifying the viewport of your views (to make something like, say, a minimap, splitscreen or something like that), this function will simply not work, because it modifies the viewports, so if that's the case you will need to find another solution.