Tableau's data blending, what Tableau does is issue two queries, one to the primary data source and one to the secondary. The query on the secondary is an aggregate query at the level of detail defined by the linking fields in the view that will only return one record per combination of the values of the linking fields, whereas a normal left join would return as many rows as were in the secondary that met the joining criteria. Once the data is returned inside Tableau, the secondary is "left joined" back to the primary.
Here linking field is returning more then one record per combination of the values. That's why you're getting "*".