Webflow's auto-generated sitemap will include all links from the Webflow site with your origin site's domain or subdomain. Proxying multiple Webflow sites or a Webflow site and your custom solutions together requires you to create and manage a custom sitemap that includes URLs from all sites and solutions.
Here are some options to consider:
- If using a proxy, use an npm package that scrapes relevant sitemaps from Webflow and your custom solutions (or two Webflow sites if you're proxying two Webflow sites together). Then it would build it into one sitemap and proxy that sitemap into place.
- Use a third-party tool such as https://pro-sitemaps.com/ to generate a complete sitemap for your site. Then you could turn off the sitemap setting within Webflow, and update the robots.txt file setting so that it has a link to the right sitemap URL. See How to customize the root domain in your sitemap.xml address.