While the 'Restore'/'Discard' last message feature is convenient, a more robust solution would be to have a Drafts folder for ticket replies. Since the 'Restore' button only restores the last message, the last message appears to get replaced with a signature on a new message.
More control over the restore feature by replacing it with reply drafts would prevent lost replies or having to type lengthy replies somewhere else (like Word with Autosave enabled) and then copy it over.