This follows from the fact that the spec has an internal slot for it:
https://w3c.github.io/webcrypto/Overview.html#dfn-CryptoKey-algorithm

Here's a test using testharness.js that would fail:

promise_test(() => {
  var algorithm = { name: "ECDSA", namedCurve: "P-256" };
  return crypto.subtle.generateKey(algorithm, false, ["sign"]).then(key => {
    assert_equals(key.publicKey.algorithm, key.publicKey.algorithm);
  });
});

Adapted from https://software.hixie.ch/utilities/js/live-dom-viewer/saved/4582

In a cursory grep of web-platform-tests this doesn't seem to be covered.