Incorrect HTTP error status codes from non-streaming generateMetadata

Link to the code that reproduces this issue

https://github.com/p00000001/error-status-codes-bug

To Reproduce

1. Start the application
2. Click the Throw notFound() page link and monitor the http status response in DevTools
3. Click the Throw redirect() page link and monitor the http status response in DevTools

Current vs. Expected behavior

• In Next.js 15, generateMetadata() became async to support metadata streaming by default: https://nextjs.org/docs/app/api-reference/functions/generate-metadata#streaming-metadata
• When streaming, I understand it is not possible to change the HTTP response code as the response headers have already been sent. Therefore, by default calling notFound() and redirect() from generateMetadata() will return a 200 response code and use client-side JavaScript to handle functionality.
• next.config.js permits a property htmlLimitedBots to block streaming until generateMetadata() is resolved.
• When streaming is blocked for a user agent, calling notFound() or redirect() from generateMetadata() should return a 404 and 307 status code respectively as streaming has not started.
• This works as expected when there is no layout.js in the root directory. However, when there is a layout.js file present, the status code is always 200 - the same behaviour as when streaming is active.
• When generateMetadata() is blocking streaming, I would have expected the same status code behaviour regardless of the presence of layout.js, as the desired status code is already known before streaming has started?
• I appreciate some crawlers can make “soft” interpretations of embedded not found page content and redirect meta tags, but this is not as reliable as http status codes, and even for crawlers that can, it could use up extra “crawl budget” processing the page.
• To test this, I have created a sample app, and changed the config to: htmlLimitedBots: /.+/ so that all user agents get blocked generateMetadata().