Hi there, daraling!
Thanks for reaching out to the Apple Support Communities! If I’m understanding your issue correctly, you have one contact that you are not getting message notifications from and I am happy to help out! You’ve tried excellent steps so far and since the issue is not resolved, I would recommend deleting the message thread by checking out this article to Use Messages on your iPhone, iPad, or iPod touch and delete the thread from this contact:
To delete a single conversation, swipe left over the conversation that you want to delete, tap Delete, then tap Delete again to confirm. Want to delete more than one conversation at a time? Open Messages and tap Edit in the upper-left corner. Then tap the circle next to the conversation and tap Delete in the bottom-right corner.
If there is information in the thread you need to save, I recommend following the steps here for How to take a screenshot on your iPhone, iPad, and iPod touch. Once you have deleted the thread, check out this article to Manage and delete contacts on your iPhone, iPad, or iPod touch to remove the contact you are having the issues with. After doing the steps here, go ahead and add the contact back and have them message you to see if the issue is resolved.
Let me know the results and have a great day!