Taxonomies with single term not showing up on node add/edit form

When I grant permissions to individual users using tac_lite by selecting more than one term in a given scheme's list (that is, the list that shows up in the user's Access by Taxonomy tab), the taxonomy select list showing the allowed terms shows up in the node add/edit form as expected. However, when I select a single term in the scheme's list, the taxonomy select list is not showing up in the node add/edit form. The taxonomy is configured to be required for the node type.

The scheme has "Visability" and "Visibility on create and edit forms" checkboxes checked.

This would not be a problem except that the single term defined in the scheme is not being assigned to the node when created. Anyone know why this is happening? If the vocabulary is required on the node type, why is the user able to create the node without populating the taxonomy select list?