You may see that the custom and standard conversions reported on Google Data Studio are different from what is showed on Facebook Ads.
Chances are the conversion window reported is different.
We encourage you to set the conversion window to 7 days click.
To do so, go to "Resource" ⏩ "Manage added data sources" ⏩ "Edit" ⏩ "Edit connection".
On the connection configurations, please choose the following options:
Conversion window to 7 days click
Report time as On conversion date
Finally, click on "Reconnect" ⏩ , then "Apply", and "Done".
Now check that this conversion window is the same that you work with on your Facebook business