RSSP - Dropdown LOV Not Displaying for Supplier and Supplier Contact Fields

In the requisition UI, the Supplier and Supplier Contact fields display a dropdown arrow. When clicked, the dropdown flashes briefly but disappears without showing any values. By contrast, fields such as Category and Agreement display a proper list of values that provides the option to select any of the values within the list.

Is this the expected behavior for Supplier and Supplier Contact fields? From a user perspective, the presence of the dropdown arrow suggests there should be an LOV available, but the UI does not display one. Should these fields only be searchable via type-ahead or the search dialog, or are they also supposed to support a dropdown LOV like Category and Agreement?