Problem with loadUi in PyQt6, wrong rendering

I'm trying to render the view through the method: uic.loadUi ("ui.ui", self), everything starts but the rendering is wrong. Below I report the code and the .ui file generated with QtDesigner.

from PyQt6 import QtCore, QtWidgets, uic
from PyQt6.QtGui import QPixmap
from PyQt6.QtWidgets import QFileDialog, QMainWindow

from Controller import Controller


class Ui_Form(QMainWindow):

    def __init__(self,controller):
        self.controller = controller
        super(Ui_Form, self,).__init__()
        uic.loadUi('ui.ui',self)
        self.show()


if __name__ == "__main__":
    

    import sys
    app = QtWidgets.QApplication(sys.argv)
    Form = QtWidgets.QWidget()
    controller = Controller()
    ui = Ui_Form(controller)
    sys.exit(app.exec())

This is the ui.ui file:

<ui version="4.0">
 <class>Form</class>
 <widget class="QWidget" name="Form">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>899</width>
    <height>779</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Form</string>
  </property>
  <layout class="QGridLayout" name="gridLayout">
   <item row="0" column="0">
    <widget class="QTabWidget" name="tabWidget">
     <property name="currentIndex">
      <number>1</number>
     </property>
     <widget class="QWidget" name="tabGeneral">
      <attribute name="title">
       <string>Tab 1</string>
      </attribute>
      <layout class="QGridLayout" name="gridLayout_2">
       <item row="0" column="0">
        <widget class="QFrame" name="frame">
         <property name="frameShape">
          <enum>QFrame::StyledPanel</enum>
         </property>
         <property name="frameShadow">
          <enum>QFrame::Raised</enum>
         </property>
         <layout class="QVBoxLayout" name="verticalLayout">
          <item>
           <widget class="QPushButton" name="addPhoto">
            <property name="text">
             <string>Add photo</string>
            </property>
           </widget>
          </item>
         </layout>
        </widget>
       </item>
       <item row="0" column="6">
        <widget class="QFrame" name="frame_2">
         <property name="frameShape">
          <enum>QFrame::StyledPanel</enum>
         </property>
         <property name="frameShadow">
          <enum>QFrame::Raised</enum>
         </property>
         <layout class="QHBoxLayout" name="horizontalLayout">
          <item>
           <widget class="QPushButton" name="back">
            <property name="text">
             <string>Back</string>
            </property>
           </widget>
          </item>
          <item>
           <widget class="QPushButton" name="next">
            <property name="text">
             <string>Next</string>
            </property>
           </widget>
          </item>
         </layout>
        </widget>
       </item>
       <item row="0" column="3" colspan="3">
        <spacer name="horizontalSpacer_2">
         <property name="orientation">
          <enum>Qt::Horizontal</enum>
         </property>
         <property name="sizeHint" stdset="0">
          <size>
           <width>324</width>
           <height>20</height>
          </size>
         </property>
        </spacer>
       </item>
       <item row="1" column="0">
        <spacer name="verticalSpacer">
         <property name="orientation">
          <enum>Qt::Vertical</enum>
         </property>
         <property name="sizeHint" stdset="0">
          <size>
           <width>20</width>
           <height>40</height>
          </size>
         </property>
        </spacer>
       </item>
       <item row="1" column="1" colspan="3">
        <widget class="QLabel" name="labelOfImage">
         <property name="minimumSize">
          <size>
           <width>500</width>
           <height>0</height>
          </size>
         </property>
         <property name="maximumSize">
          <size>
           <width>16777215</width>
           <height>16777176</height>
          </size>
         </property>
         <property name="text">
          <string>TextLabel</string>
         </property>
        </widget>
       </item>
      </layout>
     </widget>
     <widget class="QWidget" name="tabExifInfo">
      <attribute name="title">
       <string>Tab 2</string>
      </attribute>
      <layout class="QGridLayout" name="gridLayout_3">
       <item row="0" column="0">
        <widget class="QFrame" name="frame_4">
         <property name="frameShape">
          <enum>QFrame::StyledPanel</enum>
         </property>
         <property name="frameShadow">
          <enum>QFrame::Raised</enum>
         </property>
         <layout class="QGridLayout" name="gridLayout_4">
          <item row="0" column="0">
           <widget class="QListWidget" name="listExifInfo"/>
          </item>
         </layout>
        </widget>
       </item>
      </layout>
     </widget>
    </widget>
   </item>
  </layout>
 </widget>
 <resources/>
 <connections/>
</ui>

Can you help to find why it does render like this: enter image description here

Instead of this: enter image description here