Local JSON is a new feature added in version 5 which saves field group and field settings to files within your theme. The idea is similar to caching and both dramatically speeds up ACF and allows for version control over your field settings!
HOWEVER, the wp-admin field group edit screen will not load data from local json files, meaning you can not edit fields which are loaded from json files. This plugin solves this problem by creating an easy way to sync json fields into the database. This allows fields to be shared across multiple environments and contributors without having to export/import the database.
This plugin copies much of the functionality found in the Advanced Custom Fields export settings (advanced-custom-fields-pro/admin/settings-export.php).
Local Json folder filename must be the default acf-json.
Syncing down json fields will overwrite all fields stored in the database. Use with caution!