QTimer does not call connected function on timeout ramdomly

A simple project that exposes the bug can be found in the attachment.
I started a QTimer in the constructor of a derived class of QGraphicsScene,
the connected timeout function is not called randomly if line 32 in "graphics_view.h", i.e. io->setPlainText("Hello"), is called within the drawBackground function. The problem also exists if I replace that line with some other member functions of QGraphicsTextItem. And it goes away if I comment out that line. It seems that this problem only happens on mac, I did not experience same problem on Windows or Linux.