Hi Snayder,
Thank you for reaching out.
Could you confirm if the report you are using is the Audience Performance Report? If yes, you may add the Id field to SELECT clause in your query to get the Audience ID.
If you are not able to get what you are looking for after trying this, so I can further investigate, could you provide your Client customer ID and the complete report definition?
Regards,
Hiroyuki
Google Ads API Team