Custom Report API does not show data from all columns

Hi,

Hopefully you are able to help me. Some time ago the HR department created a historical custom report in Personio for us. We connect to that report via the API and ingest the data into our own data platform. But we encounter a problem with the API call. In the API call the columns that are being used in the custom report are visible:

But for the first 2 columns (first_name and last_name) no data is shown in the API. For example, when I search for first_name in Postman, I only get 2 hits:

When I search for other columns I get much more hits and also see the data. The custom report in Personio does show the columns and the values. Did anybody else encounter this problem?