Hello,
Onebox not working with this site, just show the url, i have tested this site on iframely and do not have any problem.
https://vietnamnet.vn
https://vietnamnet.vn/vn/thoi-su/chong-tham-nhung/dai-su-han-quoc-tai-viet-nam-bi-cach-chuc-vi-pham-luat-chong-tham-nhung-539726.html
meta.discourse.org and try.discourse.org also have this problem too. I take a look in to browser request and see onebox reuquest alway respone 404 code.
1. Request URL:
https://try.discourse.org/onebox?url=https%3A%2F%2Fvietnamnet.vn%2Fvn%2Fthoi-su%2Fchong-tham-nhung%2Fdai-su-han-quoc-tai-viet-nam-bi-cach-chuc-vi-pham-luat-chong-tham-nhung-539726.html&refresh=false
2. Request Method: GET
3. Status Code: 404