I do not see the 'Expanded Keys' property on the new Checkbox Tree component like the original Checkbox Tree has. I prefer the children values to be closed/collapsed, but it appears the new Checkbox Tree component expands/flattens all children by default without any way to determine the initial state.
Thanks for the feedback! I'll make sure we have a feature request on file to add this to the new component. For now, I'd recommend using the original checkbox tree