Often it’ll look like a blank line has been inserted between the quoted text and the reply, but in the plaintext email your client produces there isn’t one - this is why the raw email is useful in debugging these problems because it’ll show exactly what Discourse received and processed.
I know for sure Gmail has this behaviour, and the solution (if you don’t turn on incoming email prefer html) is to educate your users hit return twice a second time after a quote. That screenshot doesn’t look like Gmail to me, so I’m not sure if that’ll work.