Update ACF JSON load / save path in Sage 9 starter theme. Drop into app/filters.php or include from functions.php

acf-filters.php

/**
 * Update save location for ACF JSON
 */
add_filter('acf/settings/save_json', function ( $path ) {

    // Add path - Note uses get_template_directory to override any child theme JSON
    $path = get_template_directory() . '/assets/acf-json';

    // Create the directory in themes that don't exist
    if (!is_dir($path)) {
        mkdir($path);
    }

    return $path;
});

/**
 * Update load location for ACF JSON
 */
add_filter('acf/settings/load_json', function ( $paths ) {

    // Remove orginal path
    unset( $paths[0] );

    // Add path - Note uses get_template_directory to override any child theme JSON
    $paths[] = get_template_directory() . '/assets/acf-json';

    return $paths;
});