Save custom fields to custom plugin folder. All three examples do exactly the same thing in different ways.

(A) class-acf-fields-location.php

<?php

class Plugin {
	public function __construct() {
		add_filter( 'acf/settings/save_json', array( $this, 'acf_location' ) );
		add_filter( 'acf/settings/load_json', array( $this, 'acf_locations' ) ); 
	}

	public function acf_location() {
		return plugin_dir_path( __FILE__ ) . 'acf-json/';
	}

	public function acf_locations() {
		return array(plugin_dir_path( __FILE__ ) . 'acf-json/');
	}
  
}

new Plugin();

(B) acf-fields-location-simplified.php

<?php

add_filter( 'acf/settings/save_json', 'acf_location' );
add_filter( 'acf/settings/load_json', 'acf_locations' ); 

function acf_location() {
    return '/var/www/app/www/content/plugins/maatwerk-plugin/acf-json';
}

function acf_location() {
    return array('/var/www/app/www/content/plugins/maatwerk-plugin/acf-json');
}

(C) acf-fields-location-function.php

<?php

add_filter( 'acf/settings/save_json', function() {
    return '/var/www/app/www/content/plugins/maatwerk-plugin/acf-json';  
});
add_filter( 'acf/settings/load_json', function() {
    return array('/var/www/app/www/content/plugins/maatwerk-plugin/acf-json'); 
});