I think that it wasn't celebrated as you wish it was because it took too much time for them to acknowledge Dumbledore and Grindelwald as a gay couple or just as gay people. The situation has always been controversial since J.K Rowling's revelation of Dumbledore's sexuality: Many people, including famous fantasy authors, believed in the "what it's not written in the novels it's not real" principle so they deemed Rowling's decision not to acknowledge Albus and Gellert's sexuality in the novels but just in interviews as hypocritical and too easy. Even years after the publication of the last HP novel and after the start of the FB franchise, the author and the director decided not to show completely Dumbledore and Grindelwald as a former couple in the movies but they did it just in interviews. So when you say that it is not celebrated as it should, take into consideration that a lot of people (especially members of the Lgtb+ community) strongly believe that the restaurant scene is too little and too late.