These days... This has never happened before, and here we are again... When using the new template, the <NotFound> section is not applied at all. But the documentation says nothing about this. In fact, Blazor's structure changes so frequently that even the developers of the .net platform don't know what works and what doesn't. For further proof, read here: issues #4898, @SteveSandersonMS -

"@SteveSandersonMS In my view, we should remove the notfound from the template, and just return 404 letting the ASP.NET Core pipeline deal with it."

))))