Answer
The way around this is to fetch the needed value from inside the workflow itself rather than needing the trigger to supply it directly: add a Projection Task step early in the workflow that reads the shipment line and retrieves SOURCE_REF1 (or whatever field you need), then use that fetched value later in the workflow, including in a subsequent User Task notification. This lets you keep the projection-action trigger (required for the User Task to be usable at all) while still getting access to field data the action itself didn't directly expose.