Following up on my viewer observations, I downloaded Evince for Windows version 2.32 and see something that might be meaningful.

Evince has a side-panel with a drop-down for selecting Thumbnails, Index, Attachments, Layers, or Annotations. Those that are not present in the document are greyed out.

For the document with the pushpin (which Evince does display) the available choices are Thumbnails and Annotations. The Annotations selection shows the attached file on a tree under Page 1.

For the document with the hidden attachment, both the Attachments and the Annotations choices are available. When Annotations is selected, the message "Document contains no annotations" is shown. When Attachments is selected, the attachment is properly listed.

Perhaps there is something in the difference between annotations and attachments that results in different treatments by the various viewers.

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Rik