How does the Auto-Exclusion work with QRadioButtons with nested widgets?

I am building a QWidget with QRadioButtons at different levels. In other words, my widget contains some radio buttons and a subwidget (labeled groupBox in the screenshot) that also contains radio buttons.

Here is my problem: the radio buttons inside groupBox seem to interfere with the top level radio buttons (radioButton_1 and radioButton_2). I would expect that exactly one of radioButton_1 and radioButton_2 is checked at any given time, but it is now possible to uncheck these by clicking on the currently checked radio button.

The fix I came up with is to add setChecked(true) to the signal handler for radioButton_1.clicked() and radioButton_2.clicked(), but this seems a bit hacky.

connect(ui->radioButton_1, &RadioButton::clicked, [this]() {
    ui->radioButton_1.setChecked(true);
});

connect(ui->radioButton_2, &RadioButton::clicked, [this]() {
    ui->radioButton_2.setChecked(true);
});

Is there a better way to get the functionality back? Perhaps a function like setRadioButtonGroup({ui->radioButton_1, ui->radioButton_2}).

EDIT:

Per request for a MCVE, below is the form mainwindow.ui. Other files (mainwindow.cpp, main.cpp, mainwindow.h) are just the boilerplate provided when a QWidget Application is created in Qt Creator.

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <layout class="QVBoxLayout" name="verticalLayout">
    <item>
     <widget class="QRadioButton" name="radioButton_1">
      <property name="text">
       <string>radioButton_1</string>
      </property>
     </widget>
    </item>
    <item>
     <widget class="QRadioButton" name="radioButton_2">
      <property name="text">
       <string>radioButton_2</string>
      </property>
     </widget>
    </item>
    <item>
     <widget class="QGroupBox" name="groupBox">
      <property name="title">
       <string>groupBox</string>
      </property>
      <layout class="QVBoxLayout" name="verticalLayout_2">
       <item>
        <widget class="QRadioButton" name="radioButton_3">
         <property name="text">
          <string>radioButton_3</string>
         </property>
        </widget>
       </item>
       <item>
        <widget class="QRadioButton" name="radioButton_4">
         <property name="text">
          <string>radioButton_4</string>
         </property>
        </widget>
       </item>
      </layout>
     </widget>
    </item>
   </layout>
  </widget>
  <widget class="QStatusBar" name="statusBar"/>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

To address this, I suggest you create a QButtonGroup and add all four radio button to it.

See https://doc.qt.io/qt-5/qbuttongroup.html