How can we troubleshoot why a 404 page doesn't retain its URL rather changes to another 404 page of its own? I am checking for a client website for a technical SEO audit.

I am working on a client website for a technical SEO audit for the first time. They have a 404 page where the URL of the missing page is not retained, instead, it goes to a separate 404 page. For example, let's say as a user, I need to access https://www.example.com/careers , but this page is not there, so the browser redirects me too https://www.example.com/error/error/notfound?http:www.example.com. This page has all the content that a 404 page has, but the URL changes. Ideally, 404 page should not be there, that I understand, but for audit and temporary fix, shouldn't the URL have been https://www.example.com/careers and that page should show "page not found" info? How can I check, why this is happening. Also, this page is running on a .NET platform.