I've seen plenty of people mentioning they cannot trigger Beau's ending correctly. I've also had this issue when I play out his route. I manage to get Scott/Miranda's Lemon ending even though I haven't gone down either of their routes or bought the item for that plot.

I'm not sure if it's a internal bug or what, but the way it's set should have anyone get his ending after going on the final "date" with him and not taking anyone to prom. I will try to work on this and see if any corrections on my part can help with it though!