Fixed menu scroll not working right from the top, on mobile website. Works only after scrolling a certain distance

I’m facing an issue with the mobile version of the below-attached link.
When I click on the hamburger menu, and it opens, I’m unable to scroll the fixed menu right from the top. I have to scroll the whole page a bit to get the menu scroll working

*There was some custom code used to style the scroll bar, could that be creating the issue?

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