As described in 34942234 and 34919547 I am fighting with a wrong splashscreen orientation since longer.
I am pretty sure I have nailed down the root cause, which has to do with iOS 9.x (I am using iOS 9.2). When running the identical code in XCode/iOS simulator with iOS 8.4 the splashscreen is shown in Landscape orientation as expected.
Testing the entire project as .ipa-file on my iPhone 5 with iOS 9.2 provides the same problem (unfortunately I have no device with ioS 8.x installed).
My questions are: What to do next? In case of opening a support ticket with Apple - will it be free of charge in case it turns out to be a bug (which is my educated guess)?
I have opened a bug report with Apple now; will see what their answer is...