Bell with slash through it iPhone 8
Red bell with one slash through it for only one contact iPhone 8
iPhone 8, iOS 15
Where are you seeing this bell with a slash with that contact? Is this on the Contact card in the Contacts app? It would sound like you my have silenced notifications for that particular contact. If this is with a message, tap on the person's name at the top of the message page, and it will open and you would see the option to Hide Alerts. See if that is turned on.