It’s likely the custom client CRM has deduplication logic enabled/enforced.
You’ll have to determine if there is the ability to do one of the following with other app API endpoints:
Find else Create
Create/Update (aka Upsert)
Early access
Next Gen Zaps
Build and deploy automations from AI tools you already use. With Agentic Management, if anything breaks, the agent diagnoses the issue and proposes a fix.