Hey @milko,
I think this has something to do with your pinning. From the code it looks like the nav is being pinned when the top of the div is at the top of the screen. So when you open the .nav_expand to a height of 70vh, the nav div is no longer at the top of the screen. I think this is what is causing the problem. Maybe you can try putting the .nav_expand div inside the .nav (pin trigger) so that even when it expands, the .nav is still at the top of the page.