Hello,
Turns out that that is not correct, and I have been explained how it works:
The sampling decision is made by which ever agent initiates the trace and that decision is pass along to the rest of services in a header. So unfortunately, having different sampling rates for distributed traces is not possible today.
There is a small note about that behavior in https://www.elastic.co/guide/en/apm/agent/rum-js/current/distributed-tracing-guide.html, we will make sure to document that behavior better.
Sorry for my earlier mistake!