Trait proxy classes are created lazily at runtime when a trait is instantiated for the first time.
Deserializing a previously persisted session containing a proxy instance requires the
proxy class to be available to the KieBase's project class loader by the time the session is
restored. However, a freshly recreated KieBase for the purpose of restoring a session will
not have the proper classes loaded.

Note: to date, existing serialization tests reuse the same KieBase, thus not showing the issue.