It also seems weird that nobody asked this particular question. I do realize that they had other things to worry about as well (no-majes learning about magic), but nobody questioning what happened to Graves makes it look like nobody cares.

Definitely. He and Tina seemed pretty close, and I find it hard to believe that she wouldn't want to know what happened to her boss.

If he had been using polyjuice potion Graves would either have to be a prisoner or a follower of Grindelwald and he would have to be in New York so Grindelwald can make more of the potion. Now he could be anywhere and show up at any point in the sequels.

Now I'm picturing Graves popping up at some point and saying, "So this is what happens when I go on vacation."

But I like the idea that he could be anywhere.