If you do a search on this site for "testing" and click on the "Extensions" tag you'll go to https://discuss.flarum.org/t/extensions?q=testing and will only see results including that tag and that search query.
However, when I setup a new instance using beta.16 and search something and then click on a specific tag I keep seeing all search results (not only the ones from the selected tag as expected).
When I refresh the page, or just go directly to the appropriate url, it does seem to work as expected showing only the search results within the chosen tag.
I did notice when I click a tag it seems to use the api, and probably on page load the data is bound within the request so that might be the difference. Maybe there's something wrong in the api or maybe I am doing something wrong?