When you export a segment to Dynamics 365 Customer Insights - Journeys, the resulting segment will only contain contacts already in Customer Insights - Journeys that match incoming contact IDs from the Customer Insights - Data segment. Contacts in Customer Insights - Data that have IDs that aren't present in Customer Insights - Journeys (possibly because they came from another data source) will be ignored, so new contacts won't be created in Customer Insights - Journeys
So the documentation says. Does that still stand?