Customizing UINavigationBar in iOS 5 with strange behaviour

Here is a very strange iOS 5 behaviour (same in iOS 6). I'm customizing the appearance of a UINavigationBar with a UIImage as shown below.

The following code is used at application startup:

The end result is what I expect for the first 4 tabs of my UITabbar items like this:

The lighting comes from the top of the app. But, when selecting the More tab and all the other sections, the UINavigationBar look becomes flipped upside down like this:

Does somebody know why? What could be the cause of this? If you look carefully, the UIImage I'm using is actuallty flipped upside down in order to get the desired result... but when selecting the More UITabbar item and the following section, it gets displayed as it is really in the .png file but it is not the desired result as I ant the lighting to come from the top of the app.

This question was also posted on StackOverflow here: Customize UINavigationBar in iOS 5 strange behaviour