Analytics Conversations Aggregates query returns empty for custom intervals

Trying to build a custom interval report using the Genesys Cloud Analytics API. The goal is to pull conversation aggregates in 15-minute blocks for the last 24 hours. The standard getanalyticsreportconversationsaggregates endpoint works fine for predefined intervals, but switching to a custom time series query returns an empty dataset even though there’s traffic on the account.

Here’s the payload I’m sending to POST /api/v2/analytics/conversations/queries:

{
 "interval": "PT15M",
 "dateFrom": "2023-10-26T00:00:00Z",
 "dateTo": "2023-10-26T23:59:59Z",
 "groupings": ["mediaType"],
 "metrics": ["conversationCount"],
 "selections": [
 {
 "type": "equals",
 "field": "routingQueue.name",
 "value": "Support-English"
 }
 ]
}

The API returns a 200 OK, but the results array is empty. No errors in the response body. I’ve verified the queue name and date range. The same query with interval: "PT1H" or using the predefined day report type works. Is there a limitation on the number of buckets returned in a single request for custom intervals, or am I missing a required parameter for time series aggregation?