sms/imessage's allert don't work for one contact
is it possible that you have muted or limited the alert on this contact?

Or, if you have added them to the Spam or Block, then alerts would be suppressed.
https://www.igeeksblog.com/how-to-block-someone-on-imessage-on-iphone/

Open the Settings app → Scroll down and tap on Messages.
Tap on Settings then Messages on iPhone
Now, scroll down again and tap on Blocked → To check a person from this list

You can unblock them by taping on Settings → Messages → Blocked → Edit → Tap on Red minus sign → Tap on Unblock → Finally tap on Done.