the bad part is that the author cut the whole sex scene [...]
Yes, that is a pretty bad move I can agree on, we can only speculate because of that. My interpretation is that Petai was afraid and not ready, tried to refuse but couldn't resist the assertiveness of an experienced dominatrix.
That's exactly why safewords are a thing. Also, there is such thing as talking the play through beforehand so that both the sub and the dom have an idea of what's happening and agree to it.
That said, chapters 25-26 contain strong indications that Gunji has some kind of a mental disorder, causing her to flip into a different personality without remembering it. Petai clearly sees a difference in her behavior when she flips, and Gunji even has a personal psychiatrist, whom she hasn't visited in a while. To me, it all spells that she has been treating a disorder and thought it went away, but strong emotions invoked by Petai have re-triggered it all over again. If I may speculate, the cause of this disorder may be the plain old self-repression, given how Gunji seems to come from a very conservative family and thinks that her kink is something shameful and has no place in "normal" relationships, like the one she wants to have with Petai.