Grails afterInsert hook issue with Hibernate 5.1 and PostgreSQL - postgresql

Question

In an existing Grails 3.1.15 application, recently upgraded to Hibernate 5.1, an odd issue with afterInsert (or other) hooks started to appear. After some hours of testing, I could track this down to Hibernate 5.1 + PostgreSQL combination - issue is not reproducible with H2. To reproduce, create a simple application consisting of 2 domain objects - an Audit trail and a User, as shown here:
class Audit {
Date dateCreated
String auditData
}
class User {
String name
String email
def afterInsert() {
new Audit(auditData: "User created: $this").save()
}
}
The code above works OK with Hibernate 4, however, if the application is upgraded to Hibernate5 plugin + Hibernate 5.1.x (tested with 5.1.0.Final and 5.1.5.Final) the above scenario will always lead to a ConcurrentModificationException when you attempt to save a new User instance. You can just use a scaffold controller to reproduce. Note this only happens with PostgreSQL as the data source - with H2 it would still work OK.
Now, according to GORM Docs (see chapter 8.1.3) one should use a new session when attempting to save other objects in beforeUpdate or afterInsert hooks anyway:
def afterInsert() {
Audit.withNewSession() {
new Audit(auditData: "User created: $this").save()
/* no exception logged, but Audit instance not preserved */
}
}
But this wouldn't really resolve the issue with PSQL. The exception is gone, the User instance is persisted, but the Audit instance is not saved. Again, this code would work OK with H2. To really avoid the issue with PSQL, you would have to manually flush the session in afterInsert:
def afterInsert() {
Audit.withNewSession() { session ->
new Audit(auditData: "User created: $this").save()
session.flush()
/* finally no exceptions, both User and Audit saved */
}
}
Question: is this a bug, or is this expected? I find it a bit suspicious that the manual flush is required in order for the Audit instance to be persisted - and even more so when I see it works without a flush with H2 and only seems to affect PostgreSQL. But I couldn't really find any reports - any pointers are appreciated!
For the sake of completeness, I tested with the following JDBC driver versions for PostgreSQL:
runtime 'org.postgresql:postgresql:9.3-1101-jdbc41'
runtime 'org.postgresql:postgresql:9.4.1208.jre7'
runtime 'org.postgresql:postgresql:42.0.0'
And for the upgrade to Hibernate 5.1, the following dependencies were used:
classpath "org.grails.plugins:hibernate5:5.0.13"
...
compile "org.grails.plugins:hibernate5:5.0.13"
compile "org.hibernate:hibernate-core:5.1.5.Final"
compile "org.hibernate:hibernate-ehcache:5.1.5.Final"