Sharing the problem source would help to see what exactly is happening here. Perhaps one side of a comparison reduces a "1/3" to 0.3333, and then raising a negative number to that power produces no output. While the other side of the comparison is with an object where the 1/3 power is interpreted as a third root, and a negative base is OK.
You can set the interval that test points should be drawn from:
https://webwork.maa.org/wiki/FormulaTestPointshttps://webwork.maa.org/wiki/FormulaTestPoints
You will probably avoid this issue (whatever the actual issue is) if you keep the bases positive. So in this case using [-1,1] would keep cos(x) positive.