When under Setup -> Admin -> Settings -> Disable GCP Non-Project Related Cost Reallocation has been toggled to on this will disable the reallocation across both Cost History, and the GCP BigQuery Billing Export FlexReport dataset.
Null Project lines will still appear and should be filtered out with Project ID = Not Null via FlexReport filters as the Non Project Related Costs will be reassigned to a Project labelled "dummy-project-id" within the GCP BigQuery Billing Export FlexReport dataset.