Serve different cache based on GET parameter

I have content that is only available trough a newsletter, the URL in the newsletter adds a query parameter. lets say https://foobar.com/my-post/?my_query_param=12345.

If i come on the page WITH my query parameter i should be able to read the post and its content without a paywall. If i come on the page without the query parameter i should get a paywall. This works without caching.

Now i have added a dynamic caching plugin altering the cache key with the cacheaction wp_cache_key. The code looks like this:

//Get query parameter if it is present in the URL
$inputString = $_SERVER['QUERY_STRING'];
$parameters = [];
parse_str($inputString, $parameters);
//This is either INT, 0 or false.
$my_query_param = isset($parameters['my_query_param']) ? intval($parameters['my_query_param']) : false;

if( !is_user_logged_in() ) {
    if( $my_query_param === false ) {
        //No query parameter, paywall
        return $slug .= '_not_logged_in';
    } else if( $my_query_param === 0 ){
        //Invalid parameter, paywall
        return $slug .= '_not_logged_in_newsletter';
    } else{
        //Content without paywall
        return $slug .= '_not_logged_in_free_content';
    }
}

so $my_query_param can be a number, 0 or false. I get a different cache key in either situation. However a post without the query parameter gets served the same cache when i visit the same post WITH a query parameter.

I believe this is a bug but if i am missing something i would appreciate the solution.