I installed the IOS 18 beta a day ago. And right away anytime I try to respond to a text message. The app would shut down sometimes it would send the message most times it would just disappear and then if I try to read long messages, it shuts down too.
IOS 18 Beta Messages app
You need to raise each issue you find separately at https://www.apple.com/feedback/ You can post the FB numbers here if you want, so that others can link to them.