Hi mcmcmahon,
Thank you for using the Apple Support Communities! I understand you are experiencing an issue while using Messages on your iPhone. I'd like to help.
I have a few questions to gain more information.
-Are you using Messages in iCloud? Keep all your messages in iCloud - Apple Support.
-Are you seeing this same behavior on multiple devices that you are signed into, if applicable?
-Is only one of your message threads effected by this behavior?
I would like you to try the following steps. Be sure to test for resolution after each step.
-Force quit Messages: How to force an app to close on your iPhone, iPad, or iPod touch - Apple Support.
-Restart your iPhone: Restart your iPhone, iPad, or iPod touch - Apple Support.
-If only one message thread is impacted, try deleting the entire message thread from Messages: Use Messages on your iPhone, iPad, or iPod touch - Apple Support.
-Make sure your iOS is up to date under Settings > General > Software Update: Update your iPhone, iPad, or iPod touch - Apple Support.
Keep me posted on the results. Best regards.