Sticky Menu not overlaying in Landscape mode on mobile phone

Hello,
I use sticky menu with the twenty nineteen theme. When I scroll down in portrait mode everything works well on my iPhone. When I change to landscape the sticky menu appears shortly in the header zone. As soon as the header zone is passed the sticky menu hides behind the content for the rest of the page. I already set the z-index to the highest possible value without success.I tested with iPhone 6. In the responsive design mode of Safari I could see the same issue for iPhone 8 and iPhone 8 plus.

Look forward to your debugging solution.
Best regards

Ben

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