Flex Animation - insert manually for specific sections

Hi all -

I have a client side, with a header section that is set to reveal the section beneath it. The section beneath it is built to be 200vh height, with the content itself sticky, to implement this feature. Because of this, when the section scrolls into view, the flexIn class has already been applied and SS' Flex Animation doesn't execute on time.

I used script to remove and apply the class once the user has scrolled down 80vh, but the animation wasn't operating correctly. I attempted to scope SS's css manually, but it keeps fading in, I can't get the same effect to execute.

Any css ideas or solutions greatly appreciated!