What happens if I don't write event handler for PrimeNG Tree?

Question

I'm using PrimeNG Tree component inside my component. The relevant part of HTML looks like this:

  <p-tree
    [value]="allTreeNodes"
    selectionMode="checkbox"
    [filter]="true"
    (onNodeSelect)="nodeSelect($event)"
    data-testid="analysis-tree"
  ></p-tree>

As you can see, I handle onNodeSelect in my .component.ts file, but I don't handle onNodeExpand. My question is what happens when node is expanded? Does PrimeNG somehow handles it automatically? Can I somehow prove that the default event handler is run? Additionally, is there anything else I need to do from unit test for that event to reach Tree component? I tried this and the test fails at last line:

const element: DebugElement = TestUtils.findEl(fixture, 'analysis-tree');
element.triggerEventHandler('onNodeExpand', {
  node: typeNode1
});
fixture.detectChanges();
expect(typeNode1?.expanded).toBeTruthy();

I would've expected that default event handler sets expanded to true on TreeNode, but it seems I was wrong.