I noticed you were playing Master system. The Master system only has a resolution of 256×192 so needs a different scanline image. I’ve created it for you and it’s in the zip file. – Choose ‘scanline-768p-192.cfg’ and this will match the master system resolution. Master system is a perfect system resolution wise for a 768 display as 768 divides exactly into 192 (768/192=4) so there will be zero distortion with the scanlines.
If you’re playing megadrive or other systems with a screen height of 224 use the “scanline-768p-224.cfg”. This will have slight variation in the scanline height but it will mainly be noticeable on single colour backgrounds (and isn’t too bad even then).