Why doesn't get the XML printed with all elements of the child node of the ptree

The following code is used to read the xml file below

<debug>
    <filename>debug.log</filename>
    <modules>
        <module>Finance</module>
        <module>Admin</module>
        <module>HR</module>
    </modules>
    <level>2</level>
</debug>



int main()
{
    pt::ptree myTree;
    pt::read_xml("LogFormat.xml", myTree);
    pt::write_xml("RootNode.xml", myTree);

    pt::ptree& root = myTree.get_child("debug");
    pt::ptree& modulesNode = root.get_child("modules");
    pt::write_xml("ModuleNode.xml", modulesNode);
}

RootNode.xml generated was identical to the original source file.

But RootNode.xml only contained the below output

Finance

I expected it to print all the modules as below

<module>Finance</module>
<module>Admin</module>
<module>HR</module>

Can you please explain why write_xml is not consistent when generating the xml for the child node ?