This community and my own forum both show their favicon in Google and DuckDuckGo search results, and I don't think any trickery was used.
Maybe it just takes some time for the search engine to document the favicon URL?
You could always upload a separate .ico file at /favicon.ico if you want to make sure all browsers and devices find an icon as soon as possible. I think Firefox still tries to fetch that file for error pages for example.
