OK. A bug is a bug, nothing can be done here.
I was replayed the Christmas episode from the save, and there realy was dialogue with Megan. To be honest, I didn’t attach much importance to it, because I thought this episode was mostly for fun and there were no choisable options in that dialog. But then it turns out that Megan, “unplanned” by me, was brought into the story even then, although, as I said, I did nothing with her earlier.
Do I understand correctly that a relationship with this girl cannot be avoided? Is this "hardcoded" into to the story for some reason?
Anyway, thnx for the answer.