This guide applies to the Facebook Ads connector in Data Studio only, and the issue affects scorecard report elements and the "summary row" feature for tables.
There is a known issue with how Data Studio handles filters that can cause discrepancies in values between the Facebook Ads connector and the results in the Ads Manager. Instead of making the filter part of the query request to Facebook, it instead pulls the full data set and the filters post query, leading it to sum the remaining values. This is wrong, as it should have a slightly different calculation from Facebook based on that filter, but it cannot do it as it was not given the filtered data set to start. This only affects values that are calculated to be unique by Facebook, like Reach, conversion metrics, cost per X metrics, etc.
As this is on the Data Studio side, there's nothing we can do about it until Google changes the behavior. We kindly ask that you give Google feedback so they know this is a impacting issue they need to address. https://issuetracker.google.com/issues/77944627
Some users have reported that if they recreate metrics that are based off a calculation (for example, CTR or Cost per X) using the custom calculation field feature of Data Studio, they can get the real values even with a filter. This does not help for unique counts such as Reach. An example of this workaround is shown here for Google Ads but the same idea can be done for Facebook Ads.