When you use a calculated field as a dimension and select a metric (like event count), Looker Studio should re-aggregate the aggregated metrics, defaulting to the sum. It does not allow for other types of aggregation (such as average or maximum) and displays a flag indicating a data quality issue.

In this case, using the event count metric will yield correct results because it sums records for the defined event name. However, for other metrics like sessions and users, it may retrieve incorrect numbers due to differences in scope and improper aggregation.

A better alternative is to use a window function and query GA4 data in BigQuery, then connect it to Looker Studio.