Federated Data Access: Mix target data and base table data in delivery template

I am working with a situation where all the data is held in external tables (Snowflake) accessed via FDA in Adobe Campaign 7 Classic. Therefore the external recipient table (one associated with target mapping) is also a Snowflake table.  The tables are arranged in a standard relational database manner.  

If we create a delivery template where all the data comes from the base table all works well. However, as soon as we add any targetData.field  to the delivery template all the base table fields are nulled out in the delivery. Has anyone seen this before?

Is the solution to tick the box "Prepare the personalization data with a workflow" in the analysis tab of the delivery template as this seems to have fixed it?