Updating d3 force directed graph links not working

Question

I'm trying to update my force directed graph, but only the nodes are updating (not the links). I'm basically trying to update the color of the links using a css class, but it doesn't get set. The update function is called whenever something gets updated.

My code is kind of complex, so I just pasted the gist of it:

let update = () => {

    // [...]
    // Update nodes and their css classes works perfectly
    
    // But updating link css class is not updating at all

    link = link
        .data(links)
        .join(enter => {
            return enter
                .append("path")
                .attr("class", d => {

                    return "someclass"; // <-- This is not constant!
                });
        },
            update => {
                return update
            }
        );

        this._simulation.nodes(nodes);
        this._simulation.force("link").links(links);
        this._simulation.alpha(1).restart();
};

Take particular note of the return "someclass"; line. The returned string may change, but the class does not get applied.

Is there some obvious reason for this not working?

Using the following versions:

d3: 6.7.0, d3-force: 2.1.1