Symptoms

When using Choice control (SMO based, Display Type = CheckBoxList) plus "if View passes the Validation" rule it doesn't work in IE10 and older versions of IE when this view is embedded into SharePoint page as a webpart or iframe. It gives "value is required" warning even when value or values are selected in such scenario.
 

Diagnoses

This is known issue which you may see in K2 blackpearl 4.6.7/4.6.8 and respective K2 SmartForms versions. Issue manifests itself only in IE10 and older versions of IE. With IE11 and other non-Microsoft browsers it works without any issues.
 

Resolution

This is known issue with SmartForms/Control Pack for which was logged as a bug for further research. Preliminary tests in 4.6.9 RC show that there is no such issue in 4.6.9 RC - this control with validation works fine in IE10. As a workaround you may consider using standard check box list control instead of choice control, and passing values from it to another field with some replacements/adjustments.