Jasmine unit test request does not flush when testing http error as response

Question

I am attempting to flush a response with an http 400 error using the HttpTestingController . However it does not appear to ever be flushed, as the delegate of the subscription is never called.
Logging the _cancelled property of the request object indicates that returning an http error of 400 is causing the _cancelled property to be set to true. I would like an error response to be passed into the service method so that it's handling of http error codes can be tested.

The test:

  it('routes to error page when get roles receives an error', async(inject(
    [AgentManagementService, HttpTestingController],
    (service: AgentManagementService, backend: HttpTestingController) => {
      service.GetAgentPortalRoles().subscribe( (response) => {
        console.log(response);  // never gets called
        expect(1).toBe(1);
      });

      const request = backend.expectOne(environment.urlBase + 'v1/account/roles');
      expect(request.cancelled).toBeFalsy();
      console.log(request.cancelled); // is true
      request.flush( {}, { status: 400, statusText: 'Bad Request'});
      console.log(request.cancelled); // is false
    }
  )));

The function in the service being tested:

  public GetAgentPortalRoles(): Observable<Array<string>> {
    return this.http.get<RolesResponse>(API_URL + 'v1/account/roles', { observe: 'response', headers })
      .pipe(
        map( response => {
          if (response.ok) {
            return response.body.roles;
          }
          this.router.navigateByUrl('/error');
          return null;
        }),
        catchError( () => {
          this.router.navigateByUrl('/error');
          return EMPTY;
        })
      );
  }