It works fine for me - have you rebuilt your front end app? Adding it to the web.config file of the admin app does not require rebuilding. Without the key, when I use some HTML code, then e.g. the text is formatted - e.g. bold. When using the key, I see the HTML tags as plain text in the email.
Have you updated also the NuGet packages in your front end app to match the hotifix version?