The relativeURLs configuration setting has several limitations, and you’ve run into one of them.
https://gohugo.io/content-management/urls/#relative-urls
If enabled, Hugo performs a search and replace after it renders the page. It searches for site-relative URLs (those with a leading slash) associated with
action,href,src,srcset, andurlattributes. It then transforms the URL to be relative to the current page.
So, the transformation ignores stuff like this: url('/something').
I’ve written about this before. In my view a serverless site is almost never worth the cost of putting it together.