Seems to be working for me when I do the following:
- Option-drag the attachment into the note field on the Mac
- end my edit session by clicking out of the note pane
- hit sync on Mac
- hit sync on iPhone - wait a bit as the file is synced over
- tap action to open detail screen, scroll to bottom, see attachment.
Maybe the text cursor is still active in the note field, so the edit hasn't been finalized?