when I first hover over a slice, rest of the slices are displaced. In the animation see, when mouse entered the bottom right slice for the first time, other two slices moved to the left! This happens only once with 3 slices. Why does it move?

This seem to be a miscalculation of your slice coordinates.
When you hover the slice, it moves a bit out of the pie chart and the rest seems to follow. So the whole pie moves by X to the right.
Can't tell where exactly but I would check the coordinates where the pie is drawn and how the relation between the "highlighted" (moved out) slice and the rest of the pie slices works.

Edit:

Or the whole pie "jumps" to the right in your scene to react on the geometry change... You need to test it. Do you center your pie in your scene?

void Slice::mousePressEvent(QGraphicsSceneMouseEvent *e){
    if(e->button() != Qt::LeftButton) return;
    if(isSelected()) setSelected(false);
    else setSelected(true);
}
void Slice::mouseReleaseEvent(QGraphicsSceneMouseEvent*){}

Now it behaves normally:

One thing that I couldn't figure out yet is: when I first hover over a slice, rest of the slices are displaced. In the animation see, when mouse entered the bottom right slice for the first time, other two slices moved to the left! This happens only once with 3 slices. Why does it move?

Updated the github repo with these changes.

PieView::PieView(QWidget *parent) : QGraphicsView(parent){
    auto scene = new QGraphicsScene(this);
    setScene(scene);
    setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    ellipse = new QGraphicsEllipseItem();
    ellipse->setBrush(Qt::white);
    ellipse->setPen(Qt::NoPen);
    ellipse->setAcceptHoverEvents(false);
    text = new QGraphicsTextItem(ellipse);
    text->setAcceptHoverEvents(false);
    connect(this, &PieView::mouseOver, this, &PieView::onMouseOver);
    connect(this, &PieView::mouseLeave, this, &PieView::onMouseLeave);
}

and here's makePie:

void PieView::makePie(QVector<PieSeries>& series){
    scene()->clear();
    m_total = 0;
    float startAngle = 0, spanAngle;
    for (int i = 0; i < series.size(); i++) m_total += series[i].value;
    QRandomGenerator rand;
    auto rect = QRectF(0,0, 200,200);
    for (int i = 0; i < series.size(); i++){
        spanAngle = series[i].value / m_total * 360;
        auto color = QColor::fromRgb(rand.global()->bounded(0,255), rand.global()->bounded(0,255), rand.global()->bounded(0,255));
        auto slice = new Slice(startAngle, spanAngle, color, rect, series[i]);
        scene()->addItem(slice);
        startAngle += spanAngle;
    }
    auto eRect = QRectF(QPoint(sceneRect().center().x() - 75, sceneRect().center().y() - 75), QSize(150,150));
    ellipse->setRect(eRect);
    text->setTextWidth(rect.width());
}

void PieView::onMouseOver(PieSeries &s){
    text->document()->clear();
    auto cursor = QTextCursor(text->document());
    QTextBlockFormat blockFormat;
    QTextCharFormat charFormat;

    blockFormat.setAlignment(Qt::AlignHCenter);
    charFormat.setForeground(Qt::darkGreen);
    charFormat.setFontPointSize(16);
    cursor.setBlockFormat(blockFormat);
    cursor.setBlockCharFormat(charFormat);
    cursor.insertText(s.name);

    charFormat.setForeground(Qt::red);
    charFormat.setFontPointSize(14);
    cursor.insertBlock(blockFormat, charFormat);
    cursor.insertText(QString::number(s.value));

    charFormat.setForeground(Qt::black);
    charFormat.setFontPointSize(12);
    cursor.insertBlock(blockFormat, charFormat);
    cursor.insertText(QString::number(s.value / m_total * 100) + "%");

    text->setY(ellipse->boundingRect().center().y() - text->boundingRect().height()/2);
    scene()->addItem(ellipse);
}

and I remove that onMouseLeave: void PieView::onMouseLeave(){ scene()->removeItem(ellipse); }. PieView is the subclass of QGraphicsView.

EDIT
Will change that static to qobject cast later. Thanks for the reminder.

I will try later with a minimal example to test the hover behavior.
Usually it should work, unless you have done something else wrong.

Does mouseOver do something with you item?
Better use qobject_cast to cast QObjects.

auto view = static_cast<PieView*>(scene()->parent());
emit view->mouseOver(m_series);

Slice::Slice(float start, float sweep, QColor color, QRectF rect, PieSeries& series, QGraphicsItem *parent)
    : QGraphicsItem(parent), m_start(start), m_sweep(sweep), m_color(color), m_series(series)
{...}

with and without those base event calls mentioned in reply to @JonB. Didn't work.

EDIT
Here's the whole content:

#define whats16 16
Slice::Slice(float start, float sweep, QColor color, QRectF rect, PieSeries& series, QGraphicsItem *parent)
    : QGraphicsItem(parent), m_start(start), m_sweep(sweep), m_color(color), m_series(series){
    m_realColor = color;
    m_rect = rect;
    m_path = QPainterPath(QPointF(m_rect.width() / 2, m_rect.height() / 2));
    m_path.arcTo(m_rect, -m_start, -m_sweep);
    m_path.closeSubpath();
    dx = 10 * cos((m_start + m_sweep / 2) * M_PI / 180);
    dy = 10 * sin((m_start + m_sweep / 2) * M_PI / 180);
    float dimension = m_rect.width() + 2 * 20;
    m_boundingRect = QRectF(-20, -20, dimension, dimension);
    setAcceptHoverEvents(true);
    setFlag(QGraphicsItem::ItemIsSelectable);
}

EDIT

and for the working example, you can get all code of different classes/struct here

The default implementation calls update(); otherwise it does nothing.

• https://doc.qt.io/qt-5/qgraphicsitem.html#hoverEnterEvent

The default implementation will update your item. When you dont call it, it could lead to unwanted bahavior.

Do you call the base class' c'tor in Slice() while creating your Slice?
Also, like @JonB said: Pass the event to the base class in your overriden functions (call the base event handler).
E.g:

void Slice::hoverEnterEvent(QGraphicsSceneHoverEvent* ev){
// ....
QGraphicsItem::hoverEnterEvent(ev);
}

Edit:

Show your Slice constructor