The Lead Source contraint on the Lead was Created filter has a dropdown list with the same values as in the lead record's Source field dropdown list. However, when I set Lead Source constraint value to "is not empty", no leads are displayed. I am not using the Date of Activity constraint, so this should be looking at all known leads in the database. All the expected leads are displayed if I remove the Lead Source constraint form the Lead was Created filter and add the Lead Source filter.
What data is the Lead Source constraint referencing?