Workflow access attachments (photo) for use in feature description

I have a startup workflow I run to load a single feature from an ArcGIS Online hosted feature layer. Long story short, it's part of a public request process that includes a webhook to auto-email support teams a link to open and view the request in a GIS app.

I'm able to get the attachment ID on the layer using a short Arcade expression:
return First(Attachments($feature)).ID

It works fine when using the HTML5 Viewer normally using an img tag, but for some reason that particular Arcade expression apparently can't properly return the attachment ID when run as part of a startup workflow. I have other expressions to format the color and text of the feature description, and they work fine.

Any idea how to retrieve and use an attachment ID as part of a workflow? I tried Query Attachment Infos, but there didn't appear to be any way to use that output information in a Query Task or Select Features option.

PS, this is using Workflow 4, not 5.