If more than one item you could produce would bring you equally close to the desiredRatio, return the lowest index amongst the equal options. (E.G. if producing item 5 or 7 would both bring you equally close, return 5).

The tests expect the opposite.

Random tests are unacceptable: they're all of the form actualStock[i] = desiredRatio[i] + n for certain n.

You mention:

if producing item 5 or 7 would both bring you equally close, return 5

However my code returns the last item which satisfy the requirement and it passed the tests.
The description is a bit difficult to follow, I bilieve some people give up since they don't get the idea of your kata.