next set-cookie edge issue

repro-copy-headers.ts

export default function copyHeaders() {
  const headers = new Headers();
  headers.append("set-cookie", "cookie1=value1");
  headers.append("set-cookie", "cookie2=value2");

  const response = new Response(undefined, { headers });

  console.log(
    `before (length ${headers.getSetCookie().length}): `,
    headers.getSetCookie()
  );
  console.log(
    `after (length ${response.headers.getSetCookie().length}): `,
    response.headers.getSetCookie()
  );
}

In a page component (or layout etc, Node runtime is important here) there are 2 set-cookie headers in the response.
In a middleware (edge runtime) there will be 2 in "headers" and only 1 in "response".

Evens stranger, the 1 set-cookie header in "response" now contains a single invalid value: 'cookie1=value1, cookie2=value2'.

After upgrading to [email protected] the issue is fixed and there are always 2 set-cookie headers.