Detecting transient state of mandatory toOne relations is broken

The attached test-case shows that we get an TransientPropertyValueException when saving an entity, even though the referenced entity is clearly persisted before and should not be regarded as transient.
The bug seems to be introduced by and is closely related to that issue.

The new issues seems to appear when one entity is cached but the referenced entity is not (causing different DelayedPostInsertIdentifier handling for the two entities).