I think I have an idea on why its behaving like that

When the middleware runs and satisfies those conditions for alberta
-> redirects to new /AB/pathname
-> middleware runs again.. does the same thing again appending another AB to the existing AB in the url.

There needs to be a check to stop it from running it after redirection.

Maybe a check can be like :

Does the current url have AB already in it? If so, dont run the redirection