Answer
Check whether the endpoint you called belongs to an assistant/wizard-backed API rather than a plain create/update API — assistant-style endpoints stage data in a temporary "virtual" record across each step and only commit it to the real business object when the equivalent of the assistant's Finish step is called, so calling just one intermediate step's endpoint from an external integration can return success without ever persisting anything; look for a non-assistant API covering the same object for straightforward integration scenarios.