When a collection is only created after the owning entity has been persisted, a change to that collection is not made persistent.
The attached test case (run with "mvn test") produces:
When run without enhancement or when the collection is created before flush (see comment in unit test), the test produces the expected outcome.
I don't know, if this was ever supported, or whether we just relied on unsupported behaviour until now. Would you please take a look?