GA4 can count the key event while Google Ads shows zero because Ads only records eligible, attributed Google Ads conversions after the event has been imported and processed.
A sitemap opening in your browser doesn't prove Googlebot could fetch it; inspect the exact URL live and check access, redirects, robots rules, and temporary server errors.
Check that the custom domain is attached to the Pages project and its DNS is correct; if both are right, custom caching is the likely reason an old site is still being served.
Update the Production-scoped variable and make a new production deployment; existing Vercel deployments keep their original environment-variable values.