Table Grouping showing checkbox instead of dropdown carat

I have a simple table with children like shown in the docs: https://opensource.addepar.com/ember-table/docs/guides/body/rows-and-trees.
What I expected was the table with grouping and dropdown carat to indicate the expand / collapse state. but what I see is like this:

Clicking on the checkbox does collapse and expand, but what I want is an inline dropdown carat instead of checkbox

My code:

<EmberTable as |t|>
    <t.head @columns={{this.columns}} @widthConstraint='gte-container' as |h|>
      <h.row as |r|>
        <r.cell as |column|>
          {{column.name}}
        </r.cell>
      </h.row>
    </t.head>

    <t.body @rows={{this.rows}} as |b|>
      <b.row as |r|>
        <r.cell as |value|>
          {{value}}
        </r.cell>
      </b.row>
    </t.body>
  </EmberTable>

Reproduce the issue using the rows-and-trees documentation example and the provided EmberTable markup, then compare the grouping control with the screenshot. Done means grouped child rows expand and collapse through an inline dropdown carat rather than a checkbox.