Drop-down type fields are not displayed in the "select field" list in the lookup field "advanced display options" (new feature)

Drop-down type fields are not displayed in the "select field" list in the lookup field "advanced display options" (new feature)

For example, I have a "Salutation" drop down field (Dr, Mrs, Ms, etc) and this does not appear. When I change the field type to a single line text field it then appears as an option.