Has there been any more details or findings on this?
The same thing happens with my RGB modded NES when used on a XRGB3 as well. However, on the XRGB3 I can adjust the “AFC Level” setting and that fixes it. I don’t really know what that setting means, but didn’t know if adjusting it is something that could be replicated on the OSSC.
EDIT: Went and rechecked when using the XRGB3 and it seems it was cutting off some of the overscan. When I made it show all of it at the top, there was still some slight distortion even after adjusting the AFC level, but it was significantly fewer lines affected than when using the OSSC.
Using the vertical mask to hide it does make it less annoying, but then I’m annoyed (to a lesser extent) by the black bars at the top and bottom of the screen :P.