Is it a bug or why does Mia always repeat the same dialogue in this scene? Does she only have that one?
You're right, that dialogue doesn't change. It's not a bug; it's a limitation in the current game logic.
When her curiosity is below 50, she always gives the same response about looking for a book, and at this point her state is 0. Once you ask your question about having sex and she answers "yes," her state changes from 0 to 1. After that happens, there's no way to reach the next state because the conditions to trigger a state 2 were not added to the game, even though the dialogue for it was written.
I plan to fix this in the converted RPG Maker MZ AI UPSCALE version. Before that, I'm working on a few more fixes for fonts, images, and other potential bugs. Once that's ready, I'll release a smaller patch so you won't have to download the entire game again.
See the preview...