Initial Default based on category parameter not populating

Summary

Initial Default based on category parameter not populating

Content

I have written an 'Initial Default' SQL query for a requisition line DFF, which is based on the <PARAMETER.CATEGORY_ID>  provided.

However, when the line is added to the cart, the DFF does not populate with anything. 

The query only returns a single value based on category chosen, and I can see the value in the DFF drop down, but surely if this is the case then it should default?  Its not practical to ask users to have to select a single value for every line.

Is this expected?