404 status header when rewrite url variable contains %2F for slash escape

I've created a wordpress website which dynamically displays one of around 400 thousand products by passing 2 query vars to a WP page which has my custom php template. I'm using WPML to translate my pages and I've enabled the option to keep url vars _brand and _sku. All works fine with this setup (pages displays & status 200 ok header). Language switcher also keeps my query vars.

URL looks like: domain.com/en/products/?_brand=JCB&_sku=01%2F117903

however since i want the url to display like domain.com/en/products/a-brand/a-sku I've added the following rewrite rules to my child's functions.php

function rewrite_products()
{
    //en
    add_rewrite_rule('^products/([^&]+)/([^&]+)', 'index.php?page_id=93837&_brand=$matches[1]&_sku=$matches[2]', 'top');
    //nl
    add_rewrite_rule('^producten/([^&]+)/([^&]+)', 'index.php?page_id=93871&_brand=$matches[1]&_sku=$matches[2]', 'top');
    //fr
    add_rewrite_rule('^produits/([^&]+)/([^&]+)', 'index.php?page_id=93875&_brand=$matches[1]&_sku=$matches[2]', 'top');
    //de
    add_rewrite_rule('^produkte/([^&]+)/([^&]+)', 'index.php?page_id=93876&_brand=$matches[1]&_sku=$matches[2]', 'top');
    //es
    add_rewrite_rule('^productos/([^&]+)/([^&]+)', 'index.php?page_id=93877&_brand=$matches[1]&_sku=$matches[2]', 'top');
}

add_action('init', 'rewrite_products');

function rewrite_product_tags()
{
    add_rewrite_tag('%_brand%', '([^&]+)');
    add_rewrite_tag('%_sku%', '([^&]+)');
}
add_action('init', 'rewrite_product_tags', 10, 0);

I can now surf to the desired URL and I get the same page with the correct product, however the header now gives a 404 as the status.

WPML's language switcher also discards my query vars and just redirects /products/a-brand/a-sku to, for example, /producten (the translation of /products).

but most importantly, the 404 header status is not allowing me to index my pages which i have dumped in a sitemap (around 2M url's) since google thinks the url's are 404's and doesn't index those. This is in contrast is a huge problem.

I've boiled it down to the following problem:

i'm using rawurlencode which changes characters like / in the sku's to %2F
this causes the header to be a 404
JCB/005549646Z/ this url works perfect (displays fine and good header)
JCB/01%2F117903/ displays fine but has a 404 header

setting header in template doesn't work. Is my regex wrong? any help is appreciated!

Solution

I'm guessing the error was core WordPress related or somewhere in my parent theme. Anyways I never found a solution, only a workaround to replace %2F with !2F when string encoding

//Custom encoding en decoding to stop %2F 404 headers
function url_decode($encoded)
{
    $encoded = str_replace('!2F', '%2F', $encoded);
    $part_url = urldecode($encoded);

    return $part_url;
}

function url_encode($part_url)
{
    $encoded = rawurlencode($part_url);
    $encoded = str_replace('%2F', '!2F', $encoded);
    return $encoded;
}