On my Norwegian-localized forum, the “New”-link text is shown as [nb_NO.filters.new.title] when there are no new topics to view. “New (1+)” looks fine.
Make sure you don’t have any invalid translation overrides by checking /admin/customize/site_texts?q=filters.new.title. Also, test in safe mode (/safe-mode) to make sure no theme or plugin is interfering. If nothing else helps, try rebuilding your container.