Thanks for your reply.
Unfortunately in this case that is not possible because of missing features in Netlify. I already have a ticket about that on the forums and on github.
This also regards nextjs API endpoints and it’s not always possible or desired to just rewrite the API call and use SSR/SSG/ISR instead. It’s not really a solution to the question I had: why is the caching as described in the Netlify documentation not working as expected (or am I missing something)?