I have this problem on my ipad pro 12, where I thought I could only play on an iphone sized screen since there were massive black edges. It turns out that the for the ipad there are 3 pinch-zoom modes and when I pinch twice all the black bars go away and I can play fullscreen. So try to pinch your screen and zoom in to see if that fixes the problem, also in the settings have your controller size around 60-70% so that you have an easier time with the buttons. Other than that idk what else to say since it hasnt been fixed for me for almost 1.5 years.