Unpredictable behavior of QMetaMethod::invoke

#ifndef FIRSTCLASS_H
#define FIRSTCLASS_H

#include <QObject>
#include <QDebug>

class FirstClass: public QObject {
    Q_OBJECT
public:
    int firstDigit = 1;
    Q_INVOKABLE void aMethod() {qInfo() << "First" << firstDigit;}
};
#endif // FIRSTCLASS_H

#ifndef SECONDCLASS_H
#define SECONDCLASS_H

#include <QObject>

class SecondClass: public QObject {
    Q_OBJECT
public:
    int secondDigit = 2;
};
#endif // SECONDCLASS_H

#include <QCoreApplication>
#include <QMetaMethod>
#include "firstclass.h"
#include "secondclass.h"

int main(int argc, char *argv[]) {
    QCoreApplication a(argc, argv);
    FirstClass first;
    SecondClass second;
    auto firstIndex = first.metaObject()->indexOfMethod("aMethod()");
    auto firstMethod = first.metaObject()->method(firstIndex);

    firstMethod.invoke(&first, Qt::DirectConnection); // Returns True
    // Expected output: "First 1"                    | Actual output "First 1"

    firstMethod.invoke(&second, Qt::DirectConnection); // Also returns True
    // Expected output: No output? Error? "First 1"? | Actual output "First 2"

    return 0;
}