Helth Monitor http HTTP/1.1 400 Bad Request

Additionally to  suggestion, take a look at RFC3986: https://datatracker.ietf.org/doc/html/rfc3986#section-3.5

Notice the # in the URL? It is called a Fragment, typically used to identify a portion of a html document.

This is another example: https://support.f5.com/csp/article/K12783074#p2

It is linking to a headline in the solution page.

I am not exactly sure how health monitors handle the hash mark... Might be your issue.