I did find a bug in the error reporting code which may have confused you; sorry if so!

However, the actual pass/fail results of the random tests are correct and your solution is not. It was just the formatting of the error messages which was wrong; please try the following test case and observe that your function does not behave as it should:

describe("Another test", () => {
  const hole = [
      { val: '4', suit: 'Hearts' },
      { val: '6', suit: 'Hearts' }
  ];
  const flop = [
      { val: '6', suit: 'Spades' },
      { val: '9', suit: 'Spades' },
      { val: '6', suit: 'Clubs' },
  ];

  it('is a set', () => {
    assert.strictEqual(isSet(hole, flop), true);
  });
});

Issue resolved.

The random tests results are just as the tests... Random.
This is a waste of time for any Kyu level player.