To reproduce:
Create Blazor Web App (WebAssembly) in .NET 8 and choose GLOBAL interactivity location (this is the most important thing):
you will have that project structure:
Now run application and open "/error" page, firstly you will see the error page and after 1 second you will be redirected to Not found page:
The problem exists only in GLOBAL interactivity location, not in per page/component interactivity location. You can even create your own new page in Components/Pages and when you try to open that page in a browser you will be also redirected to Not found page.

