Similar code as above, but the private member variable is not being initialized to a default value:

private Set<Child> children;

@OneToMany(mappedBy = "parent")
public Set<Child> getChildren() {
  return children;
}

Bytecode instrumentation is enabled via the Maven plugin:

<plugin>
  <artifactId>maven-antrun-plugin</artifactId>
  <executions>
    <execution>
      <phase>process-classes</phase>
      <goals>
        <goal>run</goal>
      </goals>
    </execution>
  </executions>
  <configuration>
    <tasks>
      <taskdef name="instrument" classname="org.hibernate.tool.instrument.javassist.InstrumentTask">
        <classpath>
          <path refid="maven.runtime.classpath"/>
          <path refid="maven.plugin.classpath"/>
        </classpath>
      </taskdef>
      <instrument verbose="false">
        <fileset dir="${project.build.outputDirectory}">
          <include name="**/model/**/*.class"/>
        </fileset>
      </instrument>
    </tasks>
  </configuration>
</plugin>

Verified using hibernate-core versions 5.0.2.Final and 5.0.3.Final:

• first invocation of parent.getChildren() returns the correct Set instance,
• but the following invocations of parent.getChildren() all return null instead.

If the bytecode instrumentation is disabled, everything works as expected again.
Adding the fetch = FetchType.LAZY argument to the @OneToMany does not seem to make a difference.