After updating Qt from 5.9 to 5.12.3 I started getting race condition while reading qt.conf

Repro steps:

1. Create "qt.conf" file next to executable with valid configuration

2. The executable needs to start a QThread before creating an application:

int main(int argc, char** argv) {
	BackgroundThread thread{};
	thread.start();

	Application app(argc, argv);
	return app.run();
}

3. The thread needs to use "qDebug()".

class BackgroundThread : public QThread {
	Q_OBJECT
public:
	void run() override {
		qDebug() << "test";
	}
};

After running the executable several time you can observer different errors like heap corruption, assertions, and missing platform plugins message (which is most frequent).

I have attached a simple project to reproduce the behavior