iMessage app on iPhone shows 1 unread on app icon - none exists

User account in iPhone 15 Pro, iPad 12,9" 4G 3rd gen and MacBook Air M1 2020, all fully updated to latest software version.

Persisting issue: on my iPhone, the iMessage app icon has the notification of 1 unread message, yet none exists and I have checked both the iPad and MacBook which DO NOT indicate an unread message.

Has anyone else the same issue?

Could it be a malware/spy message received?

Any thoughts?

Obviously tried hard reseting the iPhone, etc. This issue also "survived" the latest iOs update which I hoped would clear the notification.