CXone Reporting API: Agent state history query returns empty for last 24h

Hit a wall trying to fetch agent state history via CXone Reporting API v2. The endpoint /api/v2/reporting/agents/statshistory returns an empty array even though agents were definitely logged in and active during the window.

Here’s the payload I’m sending:

{
 "interval": "PT1H",
 "dateFrom": "2023-10-27T00:00:00Z",
 "dateTo": "2023-10-28T00:00:00Z",
 "groupBy": "Agent",
 "select": ["agentId", "stateCode"]
}

No 4xx errors, just 200 OK with []. Is there a specific filter or scope missing for state history?