During the change of winter to summer time (Netherlands, last sunday morning of march) the time switches from 02:00 to 03:00 hours. At 3:30 (summer time), so 2:30 (original winter time) a parent and child object was stored via hibernate into the Oracle database. Technically the Calendar field in the parent is part of the key. Children objects have also this field as key.
Afterwards, retrieving the parent child objects will result that only the parent can be retrieved. In the database we verified that the calendar datetime were identical.
To repair the issue manually, I changed the datetime of the children in the Oracle database from 02:30 to 03.30. Then the parent and children both could be retrieved.