I have the same problem. Someone gets a text in Chinese (separate from texts I have actually sent) and then they take a screen shot to show me what was sent.
I have the same problem, yes. I use a SAMSUNG GALAXY 6 though, not an iPhone. I'm texting from my SMARTPHONE, not an oapp, when it happens. It's happened 3-4x, to two different friends, several months apart.
Perhaps this post will help?
There was also some mention on the web that an emoji in the text may trigger the issue, so try to mention that to your contacts.