You've encountered a known and frustrating characteristic of Real-Time Journeys segments, particularly those involving behavioral triggers like "Marketing event check-in." The issue you're describing, where new subscriptions aren't immediately reflected in the segment despite the event activity, is often related to the segment refresh and processing latency.
Understanding the Problem:
Troubleshooting and Solutions:
Key Considerations:
In summary, while there's no perfect solution to force immediate segment updates, you can try the troubleshooting steps above and provide feedback to Microsoft.