If you go here:

https://pep-previews--3649.org.readthedocs.build/pep-0742/#type-narrowing-behavior

You are taken to the desired page and anchor, but a banner is displayed with the text

This page was created from a pull request build

See the build's detail page or pull request readthedocs/readthedocs.org#3649 for more information.

That banner is helpful, but it is also in the way, so there's a little 'x' in the top right corner to click it away. However, when I click that 'x', the URL is somehow rewritten so the anchor is lost (leaving https://pep-previews--3649.org.readthedocs.build/pep-0742/#), which is not helpful.

I'm assuming this is a simple html/css/js error in the template for the banner?

(As a workaround, you can hit "Back" to go to the anchor, but that's unintuitive, and it definitely smells like a bug. :-)