Recently, while working on Trigonometry, a problem came up in which I was asked to evaluate the value of $\cos(\arccos(30^\circ))$, and I stated that the value of this function was $30$ degrees (because the cos and arccos cancel each other out). The answer, however, was stated to be undefined, as the value of $\arccos(30)$ was undefined. I plug this into Desmos, and Desmos seems to agree with that assertion, but then I plug it into Symbolab, and Symbolab seemed to agree with my answer.
What is the reason for this