Sticky Menu disappears on scrolling up (Safari)

Hi
after updating from 4.10.3 to 4.11.0 I have a strange effect on Safari-Browser, not on chrome and not on firefox: My site(s) are causing a disappearance of the sticky menu while scrolling up. (on scrolling down, the sticky menu appears/re-appears as desired).
I see in the inspector, that in <div id=”mysticky-nav” class=”wrapfixed down”> the class changes to “up”, losing the class “wrapfixed” when user scrolls up. This makes disappear the sticky menu. In other browsers “wrapfixed” is always there, together with “up” or “down”

Any Idea why Safari does behave differently to the other browsers?
How can I avoid it?

Thanks

woody18