Uncaught PHP Exception Doctrine\DBAL\Exception\UniqueConstraintViolationException: "An exception occurred while executing 'UPDATE

Hello,
I used PostgreSQL 9.3 but I executed upgrade few days ago.
Now, I am using 10.4 PostgreSQL and:
doctrine/annotations v1.2.7
doctrine/cache v1.4.2
doctrine/collections v1.3.0
doctrine/common v2.7.3
doctrine/dbal v2.5.13
doctrine/doctrine-bundle v1.5.2
doctrine/doctrine-cache-bundle v1.0.1
doctrine/inflector v1.0.1
doctrine/instantiator 1.0.5
doctrine/lexer v1.0.1
doctrine/orm v2.5.14

I have a problem with ManyToOne relation.
For example, I have main object with three child and when I execute on main object
$em = $this->getDoctrine()->getManager();
$em->merge($data);
$em->flush();
I sometimes get ERROR message like:
Uncaught PHP Exception Doctrine\DBAL\Exception\UniqueConstraintViolationException: "An exception occurred while executing 'UPDATE

I get this ERRROR message not for all main object and not for all child. For example, first update child object is ok but in second I get error.

SQL prepared by doctrine:
UPDATE child_table SET id = ?, name = ?, object_name = ?, object_size = ? WHERE id = ?' with params ["2", "test Name object 2", "test name object 2", "1234", 3]

In this sql the doctrine tries update object with id=3 using data from object with id = 2.

This problem didn't occur before executing upgrade to 10.4 version.

Can you help me and give some tips?

Pozdrawiam,
______________
Jarosław Torbicki
Analityk