All mail notifications are visible on the notification centre. However if we swipe down the notification centre immediately after ending a phone call, it shows none of the mail notifications. All mail notifications would have disappeared from the notification centre. None of the mail notifications reappear on the notification centre even by swiping it down again after waiting for five minutes. This vanishing act also happens randomly and spontaneously after some period of inactivity and none of the mail notifications are seen upon swiping down the notification centre.
The mail notifications reappear only after toggling Settings> Notifications > Mail > "Allow Notifications" settings which was On all the time, to Off and then put to On again. They also reappear randomly after some period of inactivity on the phone.
This problem continues on iOS 13.2 public release apart from all beta releases prior to this.