Newt knows Percival Graves, the real Percival Graves, isn't the same person as Grindlewald and isn't responsible for what Grindlewald did under his name, but it's difficult to remember that when Graves has so many of the same mannerisms, and looks and sounds just the same as he did before. Meanwhile, Graves is struggling with the trauma of having been kept captive for months, possibly years, by Grindlewald, and the way people he liked and respected before are now wary of him. It's not an ideal relationship, but they make it work.