Hi,
to all those still suffering with the long-standing issue of the Sticky Header migrating down the page when the Safari-using viewer scrolls back up, I did find a work-around.
Here’s the trick:
Make sure to set a Trigger Offset — in this case, I used 100 (pixels, I presume). It causes the sticky header bar to scroll off the top of the screen, but then scroll back on after the user scrolls down 100 pixels. It also causes the reverse behaviour when the user scrolls back to the top of the page.
It means the header bounces in and out, but at least it ends up where it’s supposed to be. And you can call it a little more animation eye-candy to justify it to yourself (or your client!) if you need to 
So far I’ve tried it on two sites — it seems to work whether the bar is set to Relative or Absolute position.
It’s not a 100% satisfying solution to me, but it’s about 95%, so for now… I’ll take it!
Happy building,
Carlos