Whats the correct way to set the selected value of a dropdown-menu?

No. Because actually it works under some conditions. When we initialize our view we are also doing the

private void init() {
    this.getModel().setProduct("Product A");
	...
}

And the value is set on the model and also on the real html component and this is triggering the observer correctly.
But when we do it to react on a event like here for example:

this.emitterProcessor.filter(event -> event instanceof ProductChangedEvent).subscribe(event -> {
	log.info("new event");

	this.getUI().ifPresent(ui -> ui.access(() -> {
		log.info("access UI");
		
		this.getModel().setProduct("Product C");

		log.info("new product: " + this.getModel().getProduct();
	}));
});

The output is:

new event
access UI
new product: Product C

The value is set on our model but not on our actual html component (we checked that by adding a simple clicklistener which just prints out our value set to the console), so the observer is not firing.