I can also be considered a hardcore Harry Potter fan, and I can answer this question for you as well. I don’t know if I really hate the Fantastic Beasts franchise, but I certainly don’t like some of the decisions made in the films. I think the first Fantastic Beasts film was fine. There was not much wrong with it, it introduced some very interesting new characters and it was nice seeing new magical beasts being introduced that we had not see in the previous films. And I think as 1 film that worked fine. The story also felt finished to me at the end. Except for that Grindelwald reveal at the end. But still the Fantastic Beasts film was fine with having 1 film only. And everyone would been fine with that. However problems for me started when they wanted to make a bigger franchise of it. And you didn’t know anymore what they wanted with it. Was it still about Newt and his beasts or did it become a Dumbledore story. Thhe second film was too crowded. It had too many characters, too mant storylines to focus on in a 2 hour movie. If they wanted to do more with Dumbledore than just give it a different title and no Newt, no Queenie and all those Fantastic Beasts characters. Harry Potter films worked so well because it was based on the book series. Fantastic Beasts is based on JKR’s ideas. Also she is a writer of books not a write of a film script. The 2nd film revealed that she is terrible screenwriter. So in short I liked 1 but 2 was just a bad movie.

But now to answer your question, why do some Harry Potter fans hate Fantastic Beasts ? In my opinion that is all because of CoG. All the retcons by JKR got a lot of hate and I can’t blame them for it. What I also personally hate is seeing Dumbledore, ministry workers in muggle suits. I understand that they need to blend in muggle locations, but not at the ministry or at Hogwarts. This is not Fantastic Beasts this already started back un PoA when Cuaron came with the idea of students wearing muggle clothes as well. This has all to do with fans having their own headcanon. They have their own idea how they prefer to see their favorite characters. To me it is just being faithful to the source material and they have failed to do that with Fantastic Beasts sometimes.