I'm trying to push a customer list as a Custom Audience to Facebook Ads Manager using their API (following the guide at https://developers.facebook.com/docs/marketing-api/audiences/overview. I'm encountering a weird discrepancy in audience size.

The API sync seems to be creating a significantly smaller audience compared to when I upload the same customer list as a CSV file.

Any ideas what parameter might be causing this mismatch? Anyone else run into something similar?

Thank you in advance.