Proactive message API fails with 400 for existing conversation ID

Trying to push a proactive notification to a guest using their previous conversationId. The Genesys Cloud Web Messaging docs say I can reuse the ID, but the API returns a 400 Bad Request with "reason":"invalid".

Here’s the curl call:

curl -X POST https://api.mypurecloud.com/api/v2/conversations/webmessaging/messages \
 -H "Content-Type: application/json" \
 -d '{"conversationId": "abc-123", "from": {"id": "system"}, "text": "Hello again"}'

The OTel span shows the request hits the edge correctly. Is there a specific state check required before reusing the conversation ID, or is this endpoint strictly for new sessions only?