It looks like there may be some confusion over how Google Analytics 4 (GA4) operates with the Google Tag Manager Server (GTMS). When you pass an event to GTMS, it typically sends it to GA4 via the Measurement Protocol endpoint (www.google-analytics.com/mp/collect). However, GA4 can also send this data to the gtag.js endpoint (www.google-analytics.com/g/collect) as part of its normal operation. What’s important here is payload of the request. If there’s a mismatch or if some required parameters, like session_id, are missing or differently formatted, GA4 might not interpret the data correctly. It would be a good idea to closely examine the structure and content of your initial event and compare it with what’s actually being sent to GA4. This will help you understand if there is any discrepancy causing GA4 to misunderstand the session_id. If this doesn’t solve the problem, you might need to consult GA4 documentation or seek help from the Google’s support community.