Does Apple Mail verify message has been moved to new folder before deleting it from old folder?
Through my own testing, I have been able to confirm that Apple Mail will return mail to the original folder if there are any issues moving it to the new account/folder.
At first, Apple Mail will show the mail message(s) as deleted from the original folder, then if/when an issue occurs that prevents the message(s) from being saved to the new account/folder, the message(s) will be returned to the original account/folder.