dc.js v4.1.0 and d3 v6.1.1: Uncaught TypeError: Cannot read property '2' of undefined; d is a MouseEvent

Background

We are using dc.js to display a scatter plot, but after upgrading dc.js to v4.1.0 and d3 to v.6.1.1, the tooltip is no longer displayed.

Problem

The tooltip fails here:

private tip(d: any) {
  console.log(d);
  const xPlace = this.polygons.features.find((f) => f.properties.label === d.key[2]);
}

The console displays an error:

Uncaught TypeError: Cannot read property '2' of undefined;

After upgrading, d seems to be:

MouseEvent {isTrusted: true, screenX: 790, screenY: 337, clientX: 790, clientY: 203, ...}

so obviously d.key[2] fails.

Question

Was there a breaking change that we somehow missed?

How should we resolve this issue?

Thanks!