validateOnly=true is used to test work item creation without actually creating one.
Therefore, response contains fields of not-created-workitem.
Logically, it does not contain System.ID.
That breaks WorkItem.__init__() method that tries to self.id = self.data["id"] while data has no "id".

Hello,

In a rule, have possibility to specify this directive :
.bypassrule=true

In this case, when a workitem is update, the process rules are ignored.

Thank you.