AMP-AD custom vendor request to self-hosted prebid server

Question

I have set up a custom prebid server end point here: https://pbs.cpex.cz/openrtb2/auction

Now I need the AMP-AD tag to make a request to it, from this simple demo page: https://demo.cpex.cz/amp/

The tag in question:

<amp-ad
    width="300"
    height="300"
    type="sas"
    layout="fixed"
    data-customer-name="optimics"
    data-ad-host="optimics-ads.aimatch.com"
    data-size="300x300"
    data-area="mobile-rectangle-1"
    data-site="cpexTest"
    rtc-config='{
        "URLs": [ "https://pbs.cpex.cz/openrtb2/auction?tag_id=rectangle-1" ],
        "timeoutMillis": 500
    }'>
</amp-ad>

(The data attributes are there for ad server call later on.)

According to the documentation (https://github.com/ampproject/amphtml/blob/master/extensions/amp-a4a/rtc-publisher-implementation-guide.md) I can just provide URL and the request should be made.

For some reason there is no network request to it. So I'm stuck, wondering if there is a bug, error in the documentation or my implementation.