get_bottom_checked function not working correctly?

[Simon3995]

I am using get_bottom_checked as follows:

$('#tree').on('check_node.jstree', (e, data) => {
    let tree = $('#tree');
    console.log(tree.jstree("get_bottom_checked", true));
});

When I check a node I expect to get an array of all bottom-level nodes (nodes without children). When I have a tree that looks like this:

When I uncheck and check the folder, I get an empty array (even though the child nodes do all get checked). If I uncheck and check one of the nodes within the folder I get an array of length 3, as I would expect. Am I correct in thinking this is a bug? Or am I using the function incorrectly?

p.s. I have set up checkbox like this:

checkbox: {
    tie_selection: false,
    whole_node: false,
}

so that the selection and checked status are completely independent.