Hierarchical Taxonomy Slugs/URLs are not hierarchical?

Ok, so you can make regular categories very deep:

/category/cat1/cat2/cat3/cat4/etc/
The admin menu shows them nested, and the URLs show this hierarchical structure.

With custom hierarchical taxonomies, the admin view shows them as nested, however the URLs are only 1 level deep.

The admin interface shows them correctly:

CustomTax
    TaxCategory1
    TaxCategory2
        TaxCategory2-1
        TaxCategory2-2
    TaxCategory3

The URLs should be:
/customtax/taxcategory2/ and
/customtax/taxcategory2/taxcategory2-1/

However, they are created like so:
/customtax/taxcategory2/ and
/customtax/taxcategory2-1/ (this is at the taxonomy’s ‘root’ level, instead of nested inside its parent)

… sigh… I’ve tried googling and searching this forum, to no avail.