Major REG [5.12.4 -> 5.15.0] Stylesheets ruins buttons (on Mac)

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  QString styleSheet = QStringLiteral(
        " QWidget { "
        "   background: #ebebeb;"
        " } "); 
  a.setStyleSheet(styleSheet);
  const QString msg = QStringLiteral("This does not work");
  QMessageBox::information(nullptr, QStringLiteral(""), msg);
}