I'm having the same problem using the mobile app ... still looking through the forum though. However, when using the Web app (local connection), I *don't* have this problem; the messages I send via the Web UI are kept with the original conversation on my phone. It's rather curious.
I have a Galaxy S5 running Android v5.0 (w/AT&T).
I have a Galaxy S5 running Android v5.0 (w/AT&T).