Non-html views being annotated with html comments

Description

Identified on e3e8298e locally running gdk

• Visit /rails/mailers/notify/approved_merge_request_email.txt?locale=en on your locally running GDK.
• Notice how the <!-- BEGIN and <!-- END appear. I wouldn't expect to see HTML comments in a text only view.

Investigation

• Related to !91604 (merged)
• Based on the above MR, it appears this only effects the dev environment.
• Here's the relevant view.