yes actually i did also notice this recently.....forgot we even had this addon installed, but i noticed it most where internal links to the old vb version of the site still use the favicon, but now somewhere along the way they stopped working for the main, xenforo version...
a bit suspect about the unfurled snippets being inconsistent as well but that could be a number of things still...nice to hear anyway we arent the only ones having a problem..
Now in the forums whose IDs are used above, FavIcons are no longer used and Google no longer receives metadata ...
So with us in all "private" forums ...
Perhaps it is possible for you to integrate this, and the code for internal links above in another post, into the settings of your add-on. Then it would be even more useful than it already is. It is already one of my "CoolTools" because it implements so much functionality with so much speed in XF.
That AddOn still works great for me on XF 2.2.4.
Except that that Google S2 API has become rather useless and is more often returning their default favicon rather than actual domains' favicons.
Thankfully you can easily modified this AddOn template to use GitHub favicon API instead.
var matches = link.getAttribute('href').match(/^https?\:\/\/([^\/?#]+)(?:[\/?#]|$)/i);
var domain = matches && matches; // domain will be null if no match is found
link.style.backgroundImage = "url('https://favicons.githubusercontent.com/" + domain + "')"
Actually GitHub favicon service is far from perfect either.
I had implemented some clever AJAX version that would use both services to get the best favicon but that won't work on production because of cross site security issues. Instead I did a not so clever implementation with hardcoded URL target per hostname. It can also invert favicon color if needed for those of us running dark themed forums.