The update appeared after approximately 20 days. In-app events require at least five first-time downloads from unique Apple IDs before analytics results are displayed. When I reviewed the analytics, the event showed seven first-time downloads, which explains why the data became visible at that point. Based on this behavior, each in-app event needs a minimum of five first-time downloads for results to appear in analytics.

I think this is because TestFlight disables certain sandbox flags for regulatory features. Apple might treat TestFlight builds as production like for some entitlements.