Widget inheriting from QWidget not rendering when using setFixedSize and setStyleSheet

I was working on an application based on 3 panels (status bar, side bar, content) and I found out that when I create a custom widget that inherits from QWidget and uses setFixedSize() and setStyleSheet() nothing is rendered.

Everything is fine and it renders a 1280x60 rectangle (see attached screenshot 1) using a QWidget directly:

QWidget * bar1 = new QWidget(this);
bar1->setFixedSize(1280, 60);
bar1->setStyleSheet(QString("background: #333333;"));

Whereas this renders nothing (see attached screenshot 2):

StatusBar * bar1 = new StatusBar(this);
bar1->setFixedSize(1280, 60);
bar1->setStyleSheet(QString("background: #333333;"));

And StatusBar simply inherits from QWidget without adding any code:

#include <QWidget>

class StatusBar : public QWidget
{
    Q_OBJECT

public:
    explicit StatusBar(QWidget * parent = nullptr);
};

I would expect to get exactly the same result, but apparently something is not working when inheriting from a QWidget.

A workaround is to re-implement the paintEvent() function, but this is still unexpected and wrong behavior.

I have attached an example project for reference.