Workflow Extensions - EXTERNAL_DATA field values never requested



I'm attempting to setup a workflow extension that presents a SELECT field with values from an external source. The field displays, can be updated, etc, but the "dataUrl" is never hit to request its values. I'm currently just using RequestBin to confirm contents of the requests. Are RequestBin URLs ignored?

Here's the definition, private bits excluded:
"id": redacted,
"integrationAppId": "redacted",
"extensionName": "Testing",
"webhookUrl": "",
"dataUrl": "",
"fieldMetadata": [{
"label": "EXT Field",
"key": "ext_field_1",
"fieldType": "SELECT",
"values": [{
"latestVersion": 11,
"version": 11


Looks like HubSpot is ignoring/skipping RequestBin URLs. Switching to works.