Why do you have positioner explicitly defined?

The absolute fix in my case is to mark the positioner field as @Column(insertable=false, updatable=false)

It seems that after step #3 : UPDATE CartLine set positioner=? for each CartLine in Cart, setting positioner from 0 to number of children

the positioner field in the CartLine objects are not updated by Hibernate, which may contribute to unexpected behavior when the entity is later re-merged.

During proper behavior, the chronology is:

1. INSERT INTO CartLine (child) with positioner=0
2. UPDATE Cart (parent)
3. UPDATE CartLine set positioner=? for each CartLine in Cart, setting positioner from 0 to number of children

During improper behavior, the chronology is:

1. INSERT INTO CartLine (child) with positioner=0
2. UPDATE Cart (parent)
3. UPDATE CartLine set positioner=? for each CartLine in Cart, setting positioner from 0 to number of children
4. UPDATE CartLine for each CartLine in Cart, updating all fields, and re-setting positioner to improper value

In some cases, may be related to HHH-9106.