i have never seen this problem, and i also didn´t find anything on the internet, but i have 3 ideas to try.
1: try wrapping a div around all elements under your main sticky hero slider. give this class also your background color.
2: Webflow interactions. Hide the hero if it is not in view. (little bit complicated)
3: add 2 divs as shown here
the background color of the 2nd div should be your grey not my red.
And alle your overlaying elements MUST have a z-index of at least 3. Otherwise the mask will display over the content, and so the content will dissappear.
Hope someting works
ps. i searched for “sticky content shows through div” and “sticky content flashes through overlaying content” maybe you can find something out there.