Unset Field with Filter Function not working

Hello.

When we have your plugin enabled the filter to unset a checkout field via custom function is not working.

For example we have this code

add_filter( 'woocommerce_checkout_fields' , 'custom_checkout_fields' );
function custom_checkout_fields( $fields ) {
unset($fields['billing']['billing_email']);
return $fields;
}

I know you can delete the field from your plugin but we have a custom code and now when your plug-in is enabled the unset filter is not working.

How to fix this?