I assumed it was because clicking on the tag tries to send you to https://ponerpics.org/tags/red+ribbons+(character), instead of replacing ( and ) with %28 and %29 like in every other url, but the url still doesn't work when replacing them manually.
Both work for me on the latest Pale Moon (linux64) EDIT: Believe it or not, these two have worked, but now they don't! "max (sam and max)" works though…