Web Messaging SDK: Passing CRM ID in startChat() not populating custom attributes

Docs state: “You can pass custom data to the conversation via the startChat method.”

const chat = await guestApi.startChat({
 customAttributes: {
 crmId: "CUST-12345"
 }
});

Getting 200 OK. But the crmId isn’t showing up in the webhook payload or the platform UI. Is there a specific schema requirement for customAttributes? Or does the Web Messaging Guest API strip unknown keys? Checked the API reference again, nothing mentions validation errors for custom fields.