Welcome to the wonders of http! It is a stateless protocol that requires the web application developer to write custom client-side javascript or jquery to preserve state either locally (cookies) or in session variables on the web server. Not all web application developers are asked to add this level of effort especially in a 'free' web application. It all depends on how critical the web application is.
Same thing has happened to me several times. I eventually wised up, and save my message every time by copying it to the clipboard. Occasionally, when the connection has an issue, I can go back to the clean slate and paste my message back in. It's a cumbersome process on tablets and phones, but works great on Windows.