Thank you for your post!
I was able to reproduce this issue on Mac OS X 10.10 and newer. I was unable to reproduce this behavior on the Windows Operating System -can you confirm?
Also, this does seem specific to the Header. A Top Navigation does not show the same "improper" behavior.
I don't have a version of FMP 15 under Windows installed at the moment. So unfortunatley I can't confirm the behavior under Windows.
Exactly it only affects the header.