Wrap-up code not persisting via Conversations API

Trying to set the wrap-up code for a closed interaction using POST /api/v2/conversations/{id}/wrap. The request returns 204 No Content, but the code never shows in the UI or subsequent GET calls. Here is the payload:

{
“wrapUpCode”: { “id”: “valid-code-id-from-lookup” }
}

Checked the conversation state; it’s definitely ‘closed’. Using the same bearer token that works for other updates. What am I missing?