The "(not set)" value in the "session source / medium" dimension typically indicates that there were no active sessions due to no pageviews being tracked within the configured session timeout. The default timeout setting is usually short. If the session doesn't record any pageviews within this period, the dimension may return as "(not set)". For a more detailed understanding and to adjust the session timeout settings, you can refer to the Google Analytics support page. Additionally, if you're looking for insights into specific user journeys, identifying journeys with the affected event in GA4 User Explorer or the Able Dashboard might help pinpoint the exact cause.
Able CDP released basic reporting based on first-touch attribution, which is available by clicking "Explore" button next to a chart in Able Dashboard.
For more complex use cases, tracking conversion data synced to BigQuery contains UTM parameters and referrer URLs for each event, which makes it possible to write queries that would return each known hit with UTM parameter or external referrer URL for a customer/visitor id.