What Next.js version are you on?
Apparently v14.2.7 had some fix to cache: Ensure fetch cache TTL is updated properly by ijjk 路 Pull Request #69164 路 vercel/next.js 路 GitHub
So perhaps you could start from here.
I had the same issue couple of weeks ago, popped out of the blue for our big website.
I added access token (Access Token - Documentation - Prismic) and upgraded Next to latest and so far it's not happening. And I really really hope it won't. 