I mean, his translation is trash but is there anything intentionally misleading? I haven't read the books yet but I skimmed through the heretical pope part of De Romano Pontifice and it's 100% sede so it seems he didn't touch anything.
I honestly think it's amateurish incompetence and laziness. Like I, and Simeon, said about his translation of Moral Theology: there's a lot of sections in that book which are difficult to understand because of the translation. St. Alphonsus is a very clear writer, which is reflected in his books translated by others, but this book falters because Grant simply doesn't appear to be a good Latinist. Sure, he does a great service translating these books that would otherwise be left untranslated, but that doesn't excuse the sloppiness and confusing nature of some of them.
I question if there's also a factor of it being an independent publisher problem, as many independent works and translations don't utilize editors to catch these things. I noticed this with E. Michael Jones's book Logos Rising, where he basically abuses certain sources to the point where entire sections are basically paraphrasing other works, something that I remember being a big no-no in my college history courses.