This week our production tenant (and my development tenant as well) got updated with the new 'higher' header region.
Strange enough, when hovering the flyout menus. The background was black, only the selected item was visible.
To fix this issue, you just have to switch themes: Site Settings - Change the look - select appropriate theme - Click 'try it out' - Click 'Yes, keep it'.
new green theme: issue fixed
...and back to blue: