I've known a couple of adult horses to do that- because they loved getting their butts scratched! I don't mind the "insult" in the two cases I have known, because I know the horses intentions are just to get a good scratch, but it would be a rather alarming trait if I didn't know the horses ahead of time.
Personally, when I want to get lovey-dovey with my horses, I scratch around their withers, chests and necks. They seem to like it there and you don't have a horse turning butt-first towards you. Maybe that is something to try with your filly, ignore the butt and start scratching her in the wither/neck area. They maybe you will get a front-first presentation.