A service’s spans never join the spans of the service it calls; each side starts its own trace. What is the usual cause?

Why this is the answer

A span becomes a child only if the traceparent header (or whichever propagator both sides agree on) survives the hop, so a proxy that strips it, mismatched OTEL_PROPAGATORS or a hand-rolled HTTP client that never injects context all produce two unrelated root spans. Exporter choice is invisible once data is in OTLP, sampling ratios decide what is kept rather than how spans are linked, and SpanKind is descriptive metadata, not the linking mechanism.