our app is still experiencing this issue, and what the webhook returns is 405 as the response header. We have deployed the app on amplify and I have this script in my build script
"scripts": {
"build": "rm -rf .next/cache/fetch-cache && next build",
},
405 means the method isn't allowed - ie: the type of request isn't allowed (GET,POST etc).
The revalidate route via Prismic webhooks requires a POST and it looks like Prismic sends a payload along with the request as well. Also make sure you're using /api/ in your route.
If you use anything but a POST to request the script you'll get a 405 instead.
So start with how your calling the revalidate script if its via the Prismic webhook... that should work fine.