Web Messaging Guest API: POST /api/v2/conversations/messaging/typing-indicator 400

Trying to push typing indicators from a custom desktop widget to the Genesys Cloud guest session. Sending this payload:

{"conversationId": "abc-123", "senderId": "guest-xyz", "timestamp": "2023-10-27T14:00:00Z"}

Getting a 400 Bad Request with Invalid timestamp format. We’ve used ISO 8601 for years in other endpoints. Checked the docs, nothing mentions a different format for this specific endpoint. Tried adding milliseconds. Still fails. Anyone else hit this wall with the Guest API typing indicators?