By vanishing, it means all the mail notifications showing on the notification centre disappear spontaneously without the user touching any of them and none of the mails being read by the user.
Mail app notifications on the notification centre disappear randomly:
1.) after completing a phone call; or
2.) upon picking up the phone after some period of inactivity; or
3.) clicking any one of the mail notifications on the notification centre to view that mail and then moving to home screen and then swiping down the notification centre.
Sometimes, all the mail notifications reappear upon toggle of Settings > Notifications > Mail > "Allow Notifications" from "On" to "Off" and back to "On".
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You find few notifications remain, however I find all of them vanish together from the notification centre without the user touching any of them or checking the mail.