Hi! Respectfully, this is indeed the exact image that the NES is outputting. Here’s a full frame example captured by another device: https://atariage.com/forums/topic/262587-analogue-nt-mini-color-pallete-question/
(scroll to the middle of the page).
Like I mentioned, the NES starts each scan line with a white pixel. You don’t see this on a CRT as well as many digital displays because the TV is crops the outer edges of the image (overscan).
If it bothers you, you can always crop it out manually. But to crop it out right at the capture would be undermine the goal of signal preservation, imo.
PS – the NES isn’t the only system that does this. For example a C64 also outputs a bright line: http://4.bp.blogspot.com/-p3UG3peXOMw/VH2U7a0vZZI/AAAAAAAABQk/iOW3WvSphOU/s1600/C%3D64%2BS-Video%2Bscreenshot.jpg
(again independently captured by another device, not the 2X)
Not sure why this is. Perhaps for calibration purposes.