I found the issue. In chapter 5, while visiting Emma, you have the option to show her porn or not. If you show, you have the option to have a lewd scene with Emma or to stop anything you have with her by telling her you have someone else (Lea). If you don't stop, you cheat.
I was assuming that, at the start of chapter 5, all players either cheated or broke up with Emma, and forgot that you can just not show Emma any porn to start with! In that case the game would, incorrectly, assume you're still interested in her (and have already cheated if you're Lea's girlfriend!) and present you with the dinner scene. I think that's what it's happening and it's not meant to happen.
So, I'll make another clause: if you don't show Emma porn, and you don't kiss her at the end of chapter 5, you miss your chance with her and are excluded from her future paths. In case you show her porn, you either have to break up right there or cheat on Lea. All possible paths should be covered. Sounds reasonable?
The above will also work for any other girl, like Jen. Avoiding Emma's scenes on chapter 5 and not kissing her at the end of chapter 5 will be enough to push her away from a romantic path and stay only with Jen.
The bad news is that you both will have to load an old save (once the next patch is out) and play the 2nd half of chapter 5 and the whole chapter 6 again to go to your preferred path :\ I'm sorry about that.
But thanks for bringing this up and help finding the plot holes in the story.