Symptoms

List View fields appear empty when validating.
 

Diagnoses

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

Additionally, make sure you are using the List View's SmartObject reference fields (in the context of a for loop, or when an item is added/edited/deleted) and not the Display Fields for showing List Item data, as without being in the context of a specific item, most fields won't know which List Item you're looking to return.
 

Resolution

Please see the following documentation, specifically the 'Applying Validation Patterns to Views' 'Editable List View' section:
http://help.k2.com/onlinehelp/k2forsharepoint/UserGuide/current/default.htm_Editable_List_View_Layout.html