Sorry I phrased that a bit unclearly. From what I can tell –
With backporch 15, there’s normally one black line on top and two black lines on the bottom. Sometimes the image can be moved up one, and sometimes moved up two. When it moves up two, there are four black lines on the bottom and the top line gets chopped off.
With backporch 13, there’s normally three black lines on top and no black lines on the bottom. Sometimes it moves up one, and sometimes moves up two. When it moves up two, it shows the full resolution and there’s one line top and two lines bottom.
It’s the offset needed that seems inconsistent, it’s very weird.