Strange, I'm not seeing this in my tests. Please download latest CB build release from our downloads page below and install it over the top of CB to be absolutely sure it's up to date. I can't think of any reason why it would still be applying unless there's something being inserted before the <!DOCTYPE html> as that MUST be the first line of the email for the HTML detection to work.
www.joomlapolis.com/downloads
If issue persists after updating please PM (see link in signature) backend super user login credentials and include details regarding what plan is doing this and will take a look.