widget.cpp:

#include "widget.h"
#include "ui_widget.h"
#include <QPushButton>

Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);

    auto button = new QPushButton(this);
    button->setStyleSheet("QPushButton:hover { background: Red;}");

}

widget.h:

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();

private:
    Ui::Widget *ui;
};
#endif // WIDGET_H

main.cpp:

#include "widget.h"

#include <QApplication>

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

I am using Qt creator and my Qt version is 5.14.2
My OS - MacOs Catalina

What version of Qt ?
On what platform ?
What are you getting ?
Can you provide a minimal compilable example that shows that behaviour ?