Hello tigertale,
Thank you for using Apple Support Communities and for sharing these details. You've done a great job in troubleshooting this unexpected behavior. Allow us to recommend a couple of more steps you can try. Follow the steps below:
1.) On your iPhone, locate the conversation thread and delete the conversation: Delete messages and attachments on iPhone - Apple Support
1. Open the Messages app 
on your iPhone.
2. In the Messages conversation list, swipe left on the conversation, then tap 
.
2.) Once deleted, create a new thread and send a message to that one specific contact and see if they're able to respond to you.
3.) If your issue continues, please update your iOS version. Keeping your device fully updated can help resolve various issues. Just in case, this resource can guide you: Update your iPhone or iPad - Apple Support
1.)Back up your device using iCloud or your computer.
2.)Plug your device into power and connect to the internet with Wi-Fi.
3.)Go to Settings > General, then tap Software Update.
4.) If you see more than one software update option available, choose the one that you want to install.
5.) Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do: If you forgot your iPhone passcode - Apple Support
We hope these steps help you. Thank you for using Apple Support Communities. Take care.
