guess and check

src/server/proxy.ts

@@ -42,7 +42,7 @@ export async function proxySubdomain(subdomain: string, req: Request): Promise<R
   const body = hasBody ? await req.arrayBuffer() : undefined
 
   const headers = new Headers(req.headers)
-  headers.delete('host')
+  headers.set('host', `localhost:${app.port}`)
   headers.delete('connection')
   headers.delete('keep-alive')
   headers.delete('transfer-encoding')