Same problem for me on iOS 14 with my 11 Pro Max in the UK on Virgin Mobile. I'm not in any group chats, but it seems to happen between Android and iOS a lot. I never had this problem before iOS 14 but it's a real pain as I'm not getting verification codes or messages from those who still use SMS - sometimes I just don't get the notification, but more often I don't get the messages at all, or they don't get mine.