Cancel Save Workflow not working correctly

Hello,

We have a requirement that to require users to fill in a picklist value if a certain checkbox is checked.  To meet the requirement we used the following field validation rule on the field, which works:

((FieldValue('<bcustom_ITAG>')<> 'Y') OR (FieldValue('<plcustom_ITAG>') IS NOT NULL))

However, when we use the same statement in a cancel save wf it cancels the save on any field modification.  Should the statement be modified?  Anyone have any ideas on why this WF doesn't work correctly?

Thanks!