QWizard - back button missing

I have a QWizard, which branches in a following way:

           ----- Page1 -----
         /                   \
StartPage  - Page2 - Page3 -  EndPage
         \                   /
           ----- Page4 -----

The problem is, the the end page only shows "back" button, when it's reached via StartPage -> page4 -> EndPage. Otherwise it only shows "finish" and "cancel". Does anyone have similar experiences, what could cause this? Here's how it's set up:

QWizard wizard;
wizard.setPage(0, new StartPage(&wizard));
wizard.setPage(1, new Page1(&wizard));
wizard.setPage(2, new Page2(&wizard));
wizard.setPage(3, new Page3(&wizard));
wizard.setPage(4, new Page4(&wizard));
wizard.setPage(5, new EndPage(&wizard));

For the pages, nextId() is reimplemented to return correct id. I'm confused why it works differently via different paths.

Nobody can know what is wrong because you do not show your implementation so I will just provide a code that implements the scheme that shows:

wizard.h

#ifndef WIZARD_H
#define WIZARD_H

#include <QWizard>

class QButtonGroup;

class Wizard : public QWizard
{
    Q_OBJECT
public:
    enum { Start_Page,
           Page_1,
           Page_2, Page_3,
           Page_4,
           End_Page };
    Wizard(QWidget *parent=nullptr);
};

class StartPage: public QWizardPage
{
    Q_OBJECT
public:
    StartPage(QWidget *parent=nullptr);
    int nextId() const override;
private:
    QButtonGroup *group;
};

class Page1: public QWizardPage
{
    Q_OBJECT
public:
    Page1(QWidget *parent=nullptr);
    int nextId() const override;
};

class Page2: public QWizardPage
{
    Q_OBJECT
public:
    Page2(QWidget *parent=nullptr);
    int nextId() const override;
};

class Page3: public QWizardPage
{
    Q_OBJECT
public:
    Page3(QWidget *parent=nullptr);
    int nextId() const override;
};

class Page4: public QWizardPage
{
    Q_OBJECT
public:
    Page4(QWidget *parent=nullptr);
    int nextId() const override;
};

class EndPage: public QWizardPage
{
    Q_OBJECT
public:
    EndPage(QWidget *parent=nullptr);
    int nextId() const override;
};
#endif // WIZARD_H

wizard.cpp

#include "wizard.h"

#include <QButtonGroup>
#include <QLabel>
#include <QRadioButton>
#include <QVBoxLayout>

Wizard::Wizard(QWidget *parent):
    QWizard(parent)
{
    setPage(Start_Page, new StartPage);
    setPage(Page_1, new Page1);
    setPage(Page_2, new Page3);
    setPage(Page_3, new Page3);
    setPage(Page_4, new Page4);
    setPage(End_Page, new EndPage);
}

StartPage::StartPage(QWidget *parent):
    QWizardPage(parent)
{
    QVBoxLayout *lay = new QVBoxLayout(this);
    lay->addWidget(new QLabel("Start Page"));
    group = new QButtonGroup(this);
    for(const QString & text: {"Page1", "Page2", "Page4"}){
        QRadioButton *radio = new QRadioButton(text);
        lay->addWidget(radio);
        group->addButton(radio);
    }
    group->buttons().first()->setChecked(true);
    lay->addStretch();
}

int StartPage::nextId() const
{
    if(group->checkedButton()->text() == "Page1")
        return Wizard::Page_1;
    else if(group->checkedButton()->text() == "Page2")
        return Wizard::Page_2;
    return Wizard::Page_4;
}

Page1::Page1(QWidget *parent):
    QWizardPage(parent)
{
    QVBoxLayout *lay = new QVBoxLayout(this);
    lay->addWidget(new QLabel("Page1"));
}

int Page1::nextId() const
{
    return Wizard::End_Page;
}

Page2::Page2(QWidget *parent):
    QWizardPage(parent)
{
    QVBoxLayout *lay = new QVBoxLayout(this);
    lay->addWidget(new QLabel("Page2"));
}

int Page2::nextId() const
{
    return Wizard::Page_3;
}

Page3::Page3(QWidget *parent):
    QWizardPage(parent)
{
    QVBoxLayout *lay = new QVBoxLayout(this);
    lay->addWidget(new QLabel("Page3"));
}

int Page3::nextId() const
{
    return Wizard::End_Page;
}

Page4::Page4(QWidget *parent):
    QWizardPage(parent)
{
    QVBoxLayout *lay = new QVBoxLayout(this);
    lay->addWidget(new QLabel("Page4"));
}

int Page4::nextId() const
{
    return Wizard::End_Page;
}

EndPage::EndPage(QWidget *parent):
    QWizardPage(parent)
{
    QVBoxLayout *lay = new QVBoxLayout(this);
    lay->addWidget(new QLabel("End Page"));
}

int EndPage::nextId() const
{
    return -1;
}

main.cpp

#include "wizard.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Wizard w;
    w.show();
    return a.exec();
}

The complete example can be found here.