I can think of a couple that were heavily marketed as romantic films that are really kind of a different genre, but those examples still have romance in them. I don't think I know of any that are utterly opposite of their romantic reputation.
The one's I was thinking of are "The Phantom Thread", "Punch Drunk Love" (both P.T. Anderson movies, a director who's movies are rarely marketed correctly), and, for a completely different reason, "50 Shades of Grey". It turns out that last one is a straight up dystopian nightmare.
On the reversed end, "The Village" is actually a romance marketed as a horror movie.