Hello everyone!
I wanted to join the conversation as I built the Prismic website using Next.js.
I've encountered similar issues as all of you have reported after migrating to app router. I've tried various solutions over the past few months.
However, since version 14.0.2, we haven't experienced any problems with revalidateTag() not revalidating.
The only unconventional setup we still have in place is VERCEL_FORCE_NO_BUILD_CACHE=1. As Nathan mentioned earlier, this is used to opt out of Vercel's build cache. We use it because when we trigger deploys from code changes in Git, we sometimes get outdated content if we've made content changes since the last build. It's not ideal since it doubles the build time, but now that revalidateTag() is working, our site doesn't need to rebuild as often.
Since version 14.0.4 has been released, it's possible that this issue has been fixed, as the update promises to resolve many caching issues. We're currently on the latest version, but we haven't had the chance to remove VERCEL_FORCE_NO_BUILD_CACHE=1 and test it yet.
Samuel