Noindex only in the HTTP response HTML
This means that the URL in question contains a noindex in the response HTML, and not in the rendered HTML.
Why is this important?
Rendering webpages is a resource intensive task, and it takes significantly longer than simply grabbing source (response) HTML content.
This is why Google essentially crawl URLs in a two-stage process: their 'first look' is of the HTML response, then they render the page and have a 'second look' at the rendered HTML, then they update the index based on what they found in the rendered HTML.
The 'Processing' stage is the most complex to get your head around, because things are happening in parallel. It is also necessarily more complicated, because Google are making decisions on which pages to render based on the result of the meta robots check.
Specifically, from their documentation Understand the JavaScript SEO basics;
"Googlebot queues all pages for rendering, unless a robots meta tag or header tells Googlebot not to index the page."
Essentially what this means is: noindex = no render
So this means that these pages will not get queued for rendering, and therefore Google will not see in the rendered HTML that the noindex was not present, and the page will not get indexed.
Additionally, this mismatch means it is unclear whether the page should contain noindex or not. Left unchecked, these pages will remain noindex - even if the rendering process was supposed to make them indexable.
What does the Hint check?
This Hint will trigger for any internal URL that contains a noindex robots directive in the response HTML but not in the rendered HTML.
Examples that trigger this Hint
This Hint will trigger for any URL that has meta robots=noindex in the <head> of the response HTML, that is not present in the rendered HTML.
For example, if the response HTML looked like this:
And the rendered HTML looked like this:
How do you resolve this issue?
This Hint is marked 'Critical' as it represents a fundamentally breaking issue, which may have a serious adverse impact upon organic search traffic. It is strongly recommended that Critical issues are dealt with as a matter of high priority.
You will first need to establish the desired state for these pages - if they should be noindex or not. Once this has been clarified, ensure that whatever robots directives appear in the HTML response are also the same after JavaScript rendering.