preferredStatusBarStyle var not working in iOS12?

If you choose a same status bar color for each View Controller:

<key>UIViewControllerBasedStatusBarAppearance</key>
<false/>

Ad this to your Info.plist and set status bar color from Project -> Targets -> Status Bar Style by desired color.

On the other hand, in your case, you have a navigation controller which is embedded in a view controller. Therefore, you want to different status bar color for each page.

<key>UIViewControllerBasedStatusBarAppearance</key>
<true/>

Ad this to your Info.plist. Then, create a custom class for your NavigationController. After that you can implement the method:

class LightContentNavigationController: UINavigationController {

    override var preferredStatusBarStyle: UIStatusBarStyle {
        return .lightContent
    }
}

Thats it! Please, inform me whether this was useful!

This has nothing to do with iOS 12. You just have the rules wrong.

In a navigation controller situation, the color of the status bar is not determined by the view controller’s preferredStatusBarStyle.

It is determined, amazingly, by the navigation bar’s barStyle. To get light status bar text, say (in your view controller):

self.navigationController?.navigationBar.barStyle = .black

Hard to believe, but true. I got this info directly from Apple, years ago.

You can also perform this setting in the storyboard.

Example! Navigation bar's bar style is .default:

Navigation bar's bar style is .black:

NOTE for iOS 13 This still works in iOS 13 as long as you don't use large titles or UIBarAppearance. But basically you are supposed to stop doing this and let the status bar color be automatic with respect to the user's choice of light or dark mode.