This page must have "filter" in the slug for this filter to work.
The 'filter_XXX' are tags that you can copy+paste onto resources for them to show up under the filters.
You must tag a page "filter_XXX" at least once for it to show in the filter.
E.g. if filter_XXX is listed below under the dropdown, but there's no page tagged "filter_XXX", then the option "XXX" won't show in the dropdown. There would be no point, as clicking on it would not reveal any pages in the filter results.