Test.assertEquals(highAndLow("-1 -1 0"), "0 -1"); // First failure.
Oh ok. I made wrong conclusion by error message.

The rules didn't change, you just failed different test cases. The "Expected" is what your code should have returned, but it incorrectly returned the "Actual" values instead.

I don't know which programming language you are using, but here is an example of a few test cases from the final test.

Test.assertEquals(highAndLow("1 -1"), "1 -1");
Test.assertEquals(highAndLow("1 1"), "1 1"); // Second failure.
Test.assertEquals(highAndLow("-1 -1"), "-1 -1");
Test.assertEquals(highAndLow("1 -1 0"), "1 -1");
Test.assertEquals(highAndLow("1 1 0"), "1 0");
Test.assertEquals(highAndLow("-1 -1 0"), "0 -1"); // First failure.

When sample test passes nicely, got this at actual test:
Failed asserting that two strings are equal
Expected: '0 -1'
Actual : '-1 -1'
After rearranging code got this at sample test:
Failed asserting that two strings are equal
Expected: '1 1'
Actual : '0 1'
Why it changes the rules how to represent results with equal min and max?