Why does Exercism test for state mutation?

In JS track Bird Watcher exercise Test 9 is testing for object equality. The only way for me to pass the test is to mutate the array. My initial solution where I returned a new array with the modified value triggers this test failure. It means that my map solution was not accepted. Is this intended behaviour?

const birdsPerDay = [2, 0, 1, 4, 1, 3, 0];
expect(Object.is(fixBirdCountLog(birdsPerDay), birdsPerDay)).toBe(true);

Error: expect(received).toBe(expected) // Object.is equality
Expected: true
Received: false

[arnthorsnaer]

I think those changes in description make it clearer.

[junedev]

I think in other exercises we say “returns the modified array” (or object in other contexts). That might help here as well. In general I think it makes sense to have a task that uses a for loop and mutates an array as it is a common use case.