Many iPhone users have reported issues with SMS and iMessages. When they receive an SMS or iMessage, no notifications appear. Messages can still be sent normally, but there are no alerts, no notifications, or even the red dot icon above the messaging app to indicate that there are unread messages.
It’s not just users of the iPhone 12 series experiencing this problem; even those with older iPhone models are facing similar issues. It seems that this problem stems from the iOS 14 operating system. However, not all messages are affected; some still show notifications normally, while others do not.

Apple is aware of this messaging issue but has not yet provided a solution. Even with the upcoming iOS 14.3 release next week, the message bug has been confirmed to persist. Therefore, users may have to live with this issue for a while longer.
Some users report that completely shutting down the messaging apps may temporarily resolve the issue. However, it may still return. Thus, there is currently no definitive fix available.
If you happen to forget a message from your partner or spouse, it’s not your fault, but rather Apple’s and iOS 14’s fault. So if your spouse or partner is upset because you didn’t reply to their message, let them read this article.
Source: theverge