Hey look, it’s a flutterbat!
Some things go exactly as planned; the reversal of the vampire fruit bat repellent spell worked perfectly in in “Bats!”
…But in a more realistic sense, such a drastic biological change couldn’t be so easy to remove, would it? The mind is far easier to change, but some things could just be irreversible. Still, Fluttershy takes it in stride. After all, with her semi-nocturnal sleep cycle, she can care for more critters, and make sure that even if Applejack stays out way too late catching up on whatever work she thinks she has to, she’ll at least have an opportunity to rest well with Fluttershy’s new and surprisingly warm wings wrapped snug around her like a blanket.