UINavigationBar out of sync issue

// main.m

#import <UIKit/UIKit.h>

int main(int argc, char * argv[]) {
    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, @"AppDelegate");
    }
}


// AppDelegate.m


#import <UIKit/UIKit.h>

@interface AppDelegate : UIResponder <UIApplicationDelegate>
@property (strong, nonatomic) UIWindow *window;
@end

@interface A : UIViewController
@end

@interface B : UIViewController
@end

@implementation A

- (void)viewDidLoad {
    [super viewDidLoad];

    self.navigationItem.title = @"RED";
    self.view.backgroundColor = [UIColor redColor];
}

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];

    B *b = [[B alloc] init];
    [self.navigationController setViewControllers:@[b]];
}

@end

@implementation B

- (void)viewDidLoad {
    [super viewDidLoad];

    self.navigationItem.title = @"GREEN";
    self.view.backgroundColor = [UIColor greenColor];
}

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    UINavigationController *nc = [[UINavigationController alloc] init];
    UIViewController *a = [[A alloc] init];

    [nc setViewControllers:@[a]];

    UIWindow *window = [[UIWindow alloc] init];
    window.backgroundColor = [UIColor whiteColor];
    window.autoresizingMask = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
    window.rootViewController = nc;
    [window makeKeyAndVisible];

    self.window = window;

    return YES;
}

@end