Issue
When a Nintex Form List Lookup control is connected to a Single Lines of Text SharePoint column and an item is created using Quick Edit the control is blank on the Nintex Form.
Resolution
This is expected behavior.
Additional Information
When a Nintex Form List Lookup control is linked to a Single Lines of Text SharePoint column the ID of the looked up item is recorded in the Nintex hidden NFFormData column when the form is submitted.
If the column in SharePoint is updated via Quick Edit or via a Workflow then the NFFormData column is not updated so it is not reflected on the form.