I think it is a well documented case of “jailbars”. If you type “Genesis jailbars” in Google, you should find plenty of information on this topic, maybe that’s what you’re after. As for the solution, from what I understand, right now you can mitigate the issue (make them 90% less visible… made up number btw) by modding your console (something called a “triple bypass” if I’m correct). I think Voultar is on to something and about to finally find a “100% solution” for all models. Check RetroRGB.com for more infos : http://retrorgb.com/genesisrgbbypass.html
Remember, I don’t have a Genesis, it’s just things I heard here and there. But hopefully, it will put you on the right path.