I didn't think Gindelwald would have been pretending to be him for years and years, you know?
Yeah, that was my reasoning as well once I was past the initial WTF?!-moment. Whether it was polyjuice potion or a spell, it takes years to get into the position that Graves is in and I don’t think that Grindelwald put that much effort into developing an alter-ego that he might potentially need one day. Why should he?

we shouldn't have to finish the movie and then go online to figure out what happened in the movie.
Exactly. If you need to look for answers online clearly the storytelling is somewhat lacking. It’s annoying enough with TV shows which end with cliffhangers, but at least in that case it’s done for a reason. A movie like Fantastic Beasts should be able to stand on its own though. Oh well, can’t have everything I guess.

Thanks for the link btw. Just read through the FAQ she posted and it says:
"If your burning question isn’t here, you are probably safe to assume that it will be answered in the sequels!"

Considering that she has not answered one of fandom’s most asked questions, which is “Where is Percival Graves?”, I’m not giving up hope yet that this question might be answered in one of the sequels. ;)