Currently, ReportGarden shows 0 for the demographics-related data when the data source is Facebook Ads. By demographic data, the dimensions of Age, Gender, and country.
The data that the Facebook API is returning is 0 if the end date is greater than 24th April. This is an issue from facebook's end. We have raised the ticket with the Facebook team and tracking it. https://developers.facebook.com/support/bugs/802960763982184/
You could use the corresponding (FB pixel) metric for now. For example, for metrics like Add to Cart, Purchases, use Add to Cart (FB Pixel) and Purchases (FB Pixel).