https://docs.mms.mongodb.com/reference/api/backup-configurations/

When updating the excluded namespaces, if submitting a list that includes that existing list + additions, supplying the excludedNamespaces field is enough. If, however, the list results in the removal of currently excluded namespaces, the syncSource field must be included as well. (This is because the removal necessitates a full resync.) Without the additional field, the API call will succeed, but the list will not change.