Amethyst's stone remains in her chest no matter what she changes into. Gems likely can't change where their gems are on their form.
Peetle's gem did function as an Eye, but it also was in her mouth, which did close at times. There's tons of ways that could translate to her cured state.>>78650061
Mainly cause Steven promised to help Peetle at the end of Monster Buddies.
This got people speculating on what could happen if they managed to cure (or better yet, half-cure and thus not want to do so on the other corrupted gems) Peetle and if she were to join the CGs.