After further debugging I discovered that if a entity get resolved over the 2LCache then it's currentLockMode is set to NONE.
This explains why there are already entities present in persistence-context before with lockmode NONE before transaction completition
and reflects also the behavior in previous minor Hibernate4.2.
The big question is therefore: why has Hibernate 4.3 switched implicitly to LockMode.READ where in previous releases it was NONE ?