I noticed this all the time as a developer but it exists on user websites too on their helpdesks. You go to their help subdomain and when you try to click the header link it just takes you back to the homepage of the subdomain instead of the main site.
Here’s an example: https://developers.canny.io/install
On this page there is absolutely no way to get back to the main site without retyping the url. This seems like such an obvious UX failure and yet I see it absolutely everywhere.
Edit: Another example is stripe – https://stripe.com/docs – in this case their docs don’t even exist on a subdomain