After contacting Nintex Support, it appears these are known issue but not being addressed as bugs. They occur in the latest version of Nintex Forms for SharePoint 2013 on-prem (2.9.0.0) as of Apr, 6, 2016.
Scenario 1
When updating a list item using PowerShell, list items disappear on Nintex forms. To replicate:
- create a new list
- create a new Nintex form
- attach a file to a new item in the list (the attachment shows up in the Nintex form)
- update the Title field of the newly created item using PowerShell (the attachment no longer shows in the Nintex form
Scenario 2
When adding a Nintex form to an existing list populated with items that include attachments, the attachments are not displayed on the Nintex form. To replicate:
- create a new list
- create a new item and attach a file to it
- add a new Ninitex form to the list
- try to view the list item attachments using the Nintex form (they're not displayed)
Anyone have a workaround for these issues?

