When viewing a stripped message in Outlook, attachments are not displayed or reconstructed. When viewing the same message in the Archive Manager website, the attachments can be viewed as expected.
Use MFCMapi (http://mfcmapi.codeplex.com/) to open the effected messages, they will have a blank "@OriginalMessageClass" value.
Edit the "@OriginalMessageClass" value and enter "IPM.Note" (or whatever message class the original mesage was) as this will allow the attachments to reconstruct.
Defect ST99883 has been logged for this issue.