It was rather sudden. Usually there are hints about it, or the story's obviously leaning in that direction.
There were hints, and I'm not just talking about last chapter (which started to make me really suspicious).
There was one scene in chapter 10 where Shiori-san picked our MC over all the other assistants, which seemed off, especially since they appeared reasonably close in age. Last chapter is reasonably obvious, because she was weirdly affectionate with him compared to the editors and whatnot.
However, I do think that since there were less hints than normal, and the earliest hint was subtler than normal,
there was less build-up and nearly everyone got caught by surprise.