Yup you are right, tests in JS and a couple of other languages (not all) pass -1 in some fixed tests (but not in random tests), and handling of negative numbers is not mentioned anywhere.

CodeWars showed me the inputs for the other tests.

It depends on how kata is authored. It's up to the kata author what they put in assertion messages and logs, that's why you can see inputs in some tests, and not in others.

This was in JS. Actually, I don't know that the tests were passing a negative number, only that handling them in my code solved the problem.

As much as I see, JS version does not pass negative numbers as input, and my solution does not handle negative inputs either. However, the input can be 0. Your solution could fail on 0, or had some other error. I am blind or something :|

[...] I couldn't see the actual value(s) the tests were passing to my function

You could: https://github.com/codewars/codewars.com/wiki/Troubleshooting-your-solution#how-can-i-see-which-input-causes-my-solution-to-fail

Turns out the "smallest cases" tests were passing a negative number!

In what language?