Yeah this isn't a new thing, and it is unfortunate that it happens - it's probably there to prevent people from being able to fuse multiple copies of the legendary champ in the odd case they happened to get enough copies of the rare/epic from summons.
Similarly, you can only fuse four of the epics - even if you happened to get four more rares, you can't actually fuse another epic from them.
Thanks kramaswamy.kr, that´s another good information to have.
So, I believe I´ll use the remaing goats as food or as books.