There's a workaround. If you delete the default Mail app and then reinstall it, it no longer does this. Be aware, though, that deleting the app will remove anything local that's not in the cloud. For me, I only use my iCloud email with it so it was fine. I just reinstalled it, went to my iCloud settings, and turned Mail back on. I then opened the Mail app and all my mail was downloading again. Since then, I've not seen the phantom drafts thing. Just be sure you know what you're doing with your mail before you delete the app. I can confirm, though, that this workaround has worked for me (as well as another person on this forum, who originally recommended it to me).