I have started experiencing a similar issue after I upgraded our Exchange Server 2013 to the latest Roll Up (22 I think)
Emails come through to our Outlook 2010 client, they look fine. You might reply, and that is fine. Then you might come back on to the email later on, and the entire body has turned into Chinese characters, and other weird characters.
I asked someone who can read Chinese and they said it was just gibberish, so it is not a translation of the original email from English to Chinese.
I thought this was an issue with Outlook 2010, so I have upgraded everyone to Outlook 2016 which seems to have improved the problem. But there is one particular person who is still facing the issue. It doesn't seem to affect emails
coming in from outside the business, only internal emails.