I’m having the same problem: the original page, https://avris.it/texts/roboty-do-roboty, has WARN: No metatags found, however the copy-pasted HTML code in https://avris.it/twitter.html works perfectly fine.
The HTML is exactly the same for both URLs, both have content-type: text/html, so why is one of them not handled correctly?
It still doesn’t work even if I clean up the headers in the original URL to be:
HTTP/1.1 200 OK
Date: Fri, 26 May 2017 12:02:30 GMT
Server: Apache/2.4.10 (Ubuntu)
X-Powered-By: PHP/5.5.12-2ubuntu4.6
Set-Cookie: PHPSESSID=...; path=/
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Pragma: no-cache
Vary: Accept-Encoding
Content-Type: text/html