As soon as I add a custom query_var using the filter, my pages point to an archive page - why?

Question

There are a million posts on the topic in this stack exchange, and yet for some reason I cannot get this to work for me. I wonder what I'm doing wrong?

In my functions.php I define this:

add_filter('query_vars', 'add_quote_query_vars');
function add_quote_query_vars($qv){
    $qv[] = 'xx-quote';
    return $qv;
}

Then later, on front_page.php I provide a link that embeds this into the URL:

echo  '<a href="'.add_query_arg('xx-quote', $quote->ID).'">';

Two things go wrong. 1: if, on my template page I print_r($wp_query->query_vars);, my 'xx-quote' doesn't appear in the list 2: as soon as I actually try to include that in the URL (by clicking the link, the URL is http://mysite.com/blog/?xx-quote=555) I'm taken to an archive page, rather than the expected front page.

What's interesting about #2 is that as soon as I remove the query_vars filter and click the link again, I'm taken correctly to the front page as expected.

Am I just not understanding the role of the query_vars? Is this expected behaviour? Or am I calling the filter at the wrong time (I've tried wrapping it in an init action, or even bare inside functions.php)?

I know I can just ignore all this, since I'm dealing with the front_page.php anyway, and just use $_REQUEST['xx-quote'] but that doesn't seem like a best practice at all, and I'm trying to do it "the WordPress way"...

Needless to say, the following code produces no results:

global $wp_query;
echo $wp_query->query_vars['xx-quote'];

And that is the basis of my confusion. I've read all the related posts on this SE and I seem to be doing everything right. Clearly I'm not - what's gone wrong?

Answer 1

I was experiencing a similar problem and traced it back to Yoast's WordPress SEO plugin. This is a very popular plugin, so I wanted to post my fix. If you are running this plugin, do the following:

1. Navigate to SEO > Permalinks
2. Is "Redirect ugly URL's to clean permalinks" checked?
3. If so, scroll down to the input field labelled "Other variables not to clean:" and enter the parameter(s) you wish to allow.

That fixed it for me.

Answer 2

From quick test with your filter active last test snippet works just fine for me. Also I don't get redirected from home page if there.

So it's not your approach that is wrong, but something seems to break it.