Hi all, 

I have figured out my error.  

The function creation should NOT be in the same notebook as the one used by the DLT pipeline. 

Instead, create the function separately and register it in Unity Catalog, then reference the function in the DLT pipeline.  

As a side note, UDF support for DLT is only available in DBR 13.3 and above. At the time of writing this, you will need to configure the runtime to 'preview' over 'current' to enable UDFs in DLT.