I wonder if this has something to do with the dictation issue in the main keyboard. Maybe this is an IOS issue with text boxes. When I dictate using the main keyboard in the message app, it will repeat back what I said and it all looks good. However once I send the message different words are inserted that I never said and that weren't part of what I proof read prior to hitting the send button. I first noticed this in IOS 12.1 myself.