For anyone having a similar problem, since it seems I solved it:
You have to revert do default woocommerce product page for this specific product, so that the body of the page isn’t in any way dependent on Divi modules. Or probably any theme.
You add sticky feature to additional CSS, while minding the media queries.
Since it’s my featured product, I created a custom header to which I added divi managed modules, so that there is no Divi in the body of product page.
And it works like a charm now.
