Fix draft mode invariant

packages/next/src/server/base-server.ts

@@ -2831,7 +2831,11 @@ export default abstract class Server<ServerOptions extends Options = Options> {
         res.setHeader(NEXT_DID_POSTPONE_HEADER, '1')
       }
 
-      if (isDataReq) {
+      // we don't go through this block when preview mode is true
+      // as preview mode is a dynamic request (bypasses cache) and doesn't
+      // generate both HTML and payloads in the same request so continue to just
+      // return the generated payload
+      if (isDataReq && !isPreviewMode) {
         // If this is a dynamic RSC request, then stream the response.
         if (isDynamicRSCRequest) {
           if (cachedData.pageData) {