The 'Validate when hidden' checkbox on validation rules should function in the same manner for both connected and unconnected form controls. When testing this control I noticed that validation rules fired on controls regardless of whether the 'Validate when hidden' checkbox was selected or unselected. Most of my form controls are connected to columns. When I submitted a ticket to Nintex noting that the 'Validate when hidden' checkbox didn't seem to have any affect on whether or not the validation rule triggered when a control was hidden. Nintex support has responded that this is intended functionality. In my opinion, the 'Validate when hidden' checkbox should be applicable to controls that are both connected and not connected to SharePoint columns. This would be incredibly useful.
From Nintex Support (email communication attached):
Hello Hillary,
Unfortunately, there are no external documentations regarding the 'Validate when hidden' feature.
From what I gathered from internal communications, both connected and unconnected controls were validating regardless of visibility previously. We had many requests from customers to change the behavior for unconnected controls so that they do not validate by default. The solution was to implement the 'validate when hidden' feature which only works for unconnected controls. Controls connected to an SP field will always validate regardless of visibility.
The development team has plans to hide the 'Validate when hidden' option if a control is connected to a SP field. However, it is still in the backlog with no ETA.
If you'd like to request an update on how this feature works or changes to how validation rules should be applied, I would recommend submitting an "idea" via Nintex Ideas, https://ideas.nintex.com/.
Please let me know if you have any additional questions.
Thank you,
Angela Sy
Nintex Support