According to https://developer.apple.com/documentation/foundation/nsurlsession?language=objc:
"If the response is an HTTP redirect response, the NSURLSession object calls the delegate’s URLSession:task:willPerformHTTPRedirection:newRequest:completionHandler: method. That delegate method calls the provided completion handler with either the provided NSURLRequest object (to follow the redirect), a new NSURLRequest object (to redirect to a different URL), or nil (to treat the redirect’s response body as a valid response and return it as the result)."

That means that if the client calls completionHandler(nil), the redirect request is treated as valid response and the subsequent didReceiveResponse: & didReceiveData: methods should be called. These methods are not called in case of Cronet.

As a side note, this functionality is not supported on Android either. I.e., the client can only decide whether a redirect should be followed or not. The client cannot access the content of the body.