Guillaume Smet,Sanne Grinovero, Andrea Boriero, rzo1, I've added a simpler test with classes copied out of rzo1 's test, and put them in: https://github.com/gbadner/hibernate-test-case-templates/tree/HHH-13590.

As mentioned above, the test passes on 5.4.3, and fails on 5.4.4, but only with ```hibernate.event.merge.entity_copy_observer=allow```.

Without that setting, the test fails on 5.4.3 due to ```org.hibernate.TransientObjectException: proxy was not associated with the session``` as well.

The test can probably be simplified and still reproduce the issue.