When a server uses threads, it loses the context of the context manager

[spellr]

If a server uses threads, i.e. to send requests concurrently to other servers, the threads lose the context stored in the context manager caller thread.
This means the threads won't have the context stored by the context manager, and they'll open a new trace.

[yurishkuro]

If your server is managing threads manually, the instrumentation has no way of knowing what the correct causality semantics of operations between those threads are. You may have to manage context propagation manually.