As suggested by the support I add the suggestion associated with ticket #7420116 here as well.
Please consider supplementing manuals for your products that include HTTP/Web proxies. The "Rewrite HTML" option causes not only HTML rewriting but also HTTP headers rewriting based on the head section <meta/> tags with the http-equiv attribute. The headers rewriting functionality seems to be undocumented.
Please note that such an unconditional rewriting causes problems for web pages that have a construct like the following:
Please also note that the built-in parser sometimes damages the <head/> section and moves <meta/> tags outside the section.