No, there is no hard coding of charset. I think it could be because we are applying security to prevent cross site scripting in the message.
I would temporarily set this to false in Web.config and see if it changes the result:
UseNeatHtmlForXSSPrevention
Let me know if that fixes it for you or not.
Since it is sending html email my understanding is the html content should be utf-8
Hope it helps,
Joe