Vercel subdomain rewrites look like they are caching content. I have a setup similar to https://github.com/vercel/platforms (with pages directory), and subdomain rewrites are serving different content than from visiting the path. For example, test.example.com is serving old content, and example.com/_sites/test is serving the valid new content.
Not sure what the issue is since subdomains are rerouted to the /_sites/[subdomain] path, so they should be serving the same content.