Primeng P-tree checkboxes aren’t checkable

I have a simple p-tree populated with checkboxes. I’m able to render it okay – but clicking on the checkboxes doesn’t actually make them appear as checked.

Even if I do something as simple as this in the ts file and HTML file, respectively:

ts file:

    myObjectNodes: TreeNode[] = [];

    ngOnInit() {

      const checkbox1 = {
        label:"checkbox 1"
      };

      const checkbox2 = {
        label:"checkbox 2"
      };

      this.myObjectNodes.push(checkbox1);
      this.myObjectNodes.push(checkbox2);
    }

HTML file:

    <p-tree [value]="myObjectNodes" 
              selectionMode="checkbox">

It renders fine – but the checkboxes never display as checked no matter how many times you click them:

Any suggestions?