<?php
// Save ACF fields in JSON
add_filter('acf/settings/save_json', function( $path ) {

    // update path
    $path = plugin_dir_path(__FILE__) . 'acf-json';

    return $path;
});

// Load ACF fields from JSON
add_filter('acf/settings/load_json', function( $paths ) {

    // remove original path (optional)
    unset($paths[0]);

    // append path
    $paths[] = plugin_dir_path(__FILE__) . 'acf-json';

    return $paths;
});

?>