Thanks to @shawnwall comment I realised I didn't have a Root View Controller. In the past my app given support yo iOS 3.1.3 O_O:
[self.window addSubview:self.viewController.view];
I dropped 3.1.3 support a long time ago, so I can set up a root view controller:
self.window.rootViewController = self.viewController;
That was the thing that caused the visual bug.