I have a sticky header for desktop (6em height with logo and nav) and then I have that disappear and put in a different sticky header for mobile (60px and logo). Nothing fancy about either.
If I’m on mobile, the mobile header is placed at the top of the page… until I scroll down. At that point, the header jumps down the page and appears to stick to the bottom of the invisible desktop header and stays stuck there as I scroll down the page.