【Next.js / vercel】開発環境では問題が無く、ビルド時にエラーも出ないが、デプロイ（本環境）でのデータフェッチが失敗する

export const dynamic = 'force-dynamic';

import { contentPostType } from "@/app/types/contentPostType";

export async function GET() {
    const fetchUrl: string = `${process.env.FETCH_URL}/posts`;
    const res: Response = await fetch(fetchUrl, {
        cache: 'no-store',
        headers: {
            'Content-Type': 'application/json',
            'Referer': 'https://vercelでのデプロイドメイン.vercel.app/'
        }
    });

    try {
        if (!res.ok) {
            console.error(`Response status: ${res.status}`);
            const responseText = await res.text();
            console.error(`Response text: ${responseText}`);
            throw new Error(`HTTP error! status: ${res.status}`); // vercel のログでは 403 と表示される
        }

        const data: contentPostType[] = await res.json();
        return Response.json(data, { status: 200 });
    } catch (error: any) {
        console.error('Fetch error:', error);
        return Response.json({ error: error.message, stack: error.stack }, { status: 500 });
    }
};

export const dynamic = 'force-dynamic';

import { GET } from "./api/posts/route"
import baseStyles from "./styles/page.module.css";
import Link from "next/link";
import { notFound } from "next/navigation";
import { contentPostType } from "./types/contentPostType";
import PostArchive from "./components/elements/PostArchive";

export default async function Home() {
  const res: Response = await GET();
  const postsFetchData_contentType: string | null = res.headers.get('content-type');

  const jsonResponse = JSON.stringify(res);
  console.log(`Response size: ${Buffer.byteLength(jsonResponse, 'utf8')} bytes`);

  try {
    if (
      postsFetchData_contentType &&
      !postsFetchData_contentType.includes('application/json')
    ) {
      console.error(`app/page.tsx --- Response status: ${res.status}`); // フロント側のログに 500 が表示される
      console.error(`app/page.tsx --- Response text: ${await res.text()}`);
      throw new Error("src/app/page.tsx - postsFetchData");
    }
  } catch (error) {
    console.error("An error occurred:", error);
  }

  if (res.status === 404 || res.status === 500) notFound();
  const postsData: contentPostType[] = await res.json();

  return (
    <main>
      <section className={baseStyles.sectionElm}>
        <h2 className={baseStyles.sectionHeading_2}>お知らせ</h2>
        <Link href={'/posts'}>お知らせ一覧へ</Link>
        <div className={baseStyles.infoBox}>
          <PostArchive props={{
            fetchData: postsData,
            archivePathName: 'posts'
          }} />
        </div>
      </section>
    </main>
  );
}