With the new top nav of English SO this bug is more or less disguised, but it is more present in parts of the network that still have black top nav.
When you do a search by tag, if you're in
/questions it does it well but when you are in
/unanswered, and you want to see a search by tag this sort of works, but redirects you using
/questions (e.g. https://stackoverflow.com/questions/tagged/) instead of
/unanswered (e.g. https://stackoverflow.com/unanswered/tagged/) because what you want to see are unanswered questions filtered by tag, not all questions by tag.
Although maybe it is not a bug and it is the expected behavior, something confusing has my understanding, but maybe just as it is not a bug.
I think the confusion or perception of strange behavior comes because in the places of the network they use this system, the old nav:
Questions Tags Users Badges Unanswered
Then you have access to Unanswered, when doing a search for a tag, I would expect that filter applied directly in the section where I am, not that I redirect to the questions part and I have to navigate back.
But within English SO's new top nav (Questions Jobs Documentation BETA Tags Users) there is no Unanswered, which clears up that possible confusion, bug, or strange behavior, in my opinion.