That IS the long haired senpai/chief, she just has cut her hair short in the story somewhere in the mid 40 chapters. They were on a business trip, both got very drunk, and it sort of just happened, but yes FMC's response is interesting. In later chapters he is also contemplating the complexity of the relationships. They were always quite explicit that they are not exclusive or committed or lovers, so this could be a plot device going forward. You gotta remember this is an alternate timeline serialization based on the artist's web comic she released on Twitter before COVID, which is still available here:
https://mangadex.org/title/0e372504-2af6-4699-8abc-8eba9c9cce1e/friends-with-benefits