Symptoms

Due the the way Editable List Views are handled, validation will need to happen as items are added to the list rather than at the very end. Adding validation at the point where a list item is added will ensure that all required fields of an item are entered before further processing.

The following documentation has some helpful screenshots regarding this, specifically the 'Applying Validation Patterns to Views' 'Editable List View' section:
http://help.k2.com/onlinehelp/K2Appit/UserGuide/current/default.htm#List_View_Layout.html
 

Diagnoses

Due the the way Editable List Views are handled, validation will need to happen as items are added to the list rather than at the very end. Adding validation at the point where a list item is added will ensure that all required fields of an item are entered before further processing.
 

Resolution

The following documentation has some helpful screenshots regarding this, specifically the 'Applying Validation Patterns to Views' 'Editable List View' section:
http://help.k2.com/onlinehelp/K2Appit/UserGuide/current/default.htm#List_View_Layout.html

I wrote a very similar community post detailing much of this: http://community.k2.com/t5/K2-blackpearl/List-View-fields-appear-empty-when-validating/ta-p/90731