No options shown when setting default value of Existing Contact from Relationship to.

1.0 Problem/Motivation
Apologies if this is a known todo list item.

I have a webform with two contacts with a Relationship between them. Am trying to set the default value of Existing Contact field for Contact 2 from the Relationship to Contact 1. In the Edit Existing Contact pop-up form I can select Set Default Contact from Relationship to..., but the Specify Relationships listbox just says 'No Relationship Types defined for Household to Individual',

Has some similarity to https://www.drupal.org/project/webform_civicrm/issues/3153274 → , but I think it's a different circumstance?

2.0 Detailed steps to reproduce (embed screenshots)
This is on latest download from Github branch 8.x-5.x.

Attachment Relationships 1 shows the configuration screen for the field options.

Attachment Relationships 2 shows the configuration of the Relationship.

3.0 Proposed resolution
in CivicrmContact.php, Line 290 in the function form(), I can see that $form['contact_defaults'] is being set up as expected, given the screen displayed (attachment Relationships 3), and also that $element_properties['default_relationship'] is empty, which corresponds (Relationships 4).

I will try and investigate further if you could give me some guidance on where to look next (or whether I am going in the wrong direction).