Return a list of records, but filtered by unique values of one field

I am using the create csv from json step shown below.

In the data type, there is a field for an external ID. Each unique External ID can have more than one record in this data type. When the CSV is created, I do not want to see the same External ID on more than one row. I am using the list of unique records to report several other columns on the csv so I do need to have a list of records and not just a list of unique External IDs.

I thought I had a solution using an advanced feature, but it is returning under 500 records when I know there are about 1,500 records that should be returned? Seems strange that it appears to be working for about 1/3 of the records, but not the complete list? This is all being done in the backend.

Anyone have any ideas on why this is not working or if you know of a different way to get the outcome I am looking for?

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did you ever figure this out? i’m trying to do this too.