Perhaps some nouns are used both as animate (Nom=Gen) and inanimate (Nom=Accus), depending on the context.
Like the "добавить в друзья" in social networks. Here the word "friends" is used in the sense of "friend list", not of the humans themselves.
As such, "в гостях" reflects the prepositional usage of в, while "в гости" reflects the accusative usage of a noun which, although derived from the word for "guests", here implies a change in quality/state, rather than the embodiment of the guests themselves.
By the way, remember that лицо is inanimate when meaning "face" and animate as a formal word for "person". Which implies an animate accusative plural in "лиц". Like in the idiom "мера на лиц...", "a measure, concerning [a group of specific] people".