Polylang breaks custom query var

Polylang free version 1.9.3
WordPress 4.5.3

Consider the following code:

add_action( 'init', 'create_child_pages_permalinks' );
function create_child_pages_permalinks(){
	add_rewrite_rule(
		'services/([0-9]+)/?$',
		'index.php?pagename=services&child_page_tax=test_query_var',
		'top'
	);
}

add_filter( 'query_vars', 'child_page_tax_query_var' );
function child_page_tax_query_var( $query_vars ){
	$query_vars[] = 'child_page_tax';
	return $query_vars;
}

Then, on page.php;

var_dump(get_query_var('child_page_tax'));

With Polylang disabled, I get ‘test_query_var’. With Polylang enabled, I get an empty string. For some reason, Polylang is preventing me from adding a custom query var.

The real life purpose of this code is to create permalinks with a structure like /{parent_page}/{taxonomy_term}.

https://ww.wp.xz.cn/plugins/polylang/