lupdate ignores strings if .cpp file includes other .cpp files

The attached example application demonstrates the use case. Source file
myclass.cpp includes other .cpp files i.e. myclass1.cpp and myclass2.cpp. All of these .cpp files implement functions for MyClassTest:

myclass.cpp:

#include "myclass.h"

MyClassTest::MyClassTest(QObject *parent) :
    QObject(parent)
{
}

QString MyClassTest::doTranslate()
{
    QString test(tr("My test:"));
    return test;
}

#include "myclass1.cpp"
#include "myclass2.cpp"

myclass1.cpp:

#include "myclass.h"

QString MyClassTest::doTranslate1()
{
    QString test1(tr("My test1:"));
    return test1;
}

--> when lupdate is run for test.pro the strings on myclass1.cpp and myclass2.cpp are not added to the generated .ts file.
Note: if .cpp includes are removed from myclass.cpp the lupdate picks up the strings also from myclass1.cpp and myclass2.cpp.