Filter response is changed according to its order
An order of 30 is way too high, especially for a band reject filter with such a narrow band as you want.

Your filter will have 15 poles in a very small semicircle between frequencies 58 and 62 Hz (and another 15 poles in the negative frequencies).

Your a coefficients correspond to an order 30 polynomial with the 30 roots corresponding to the poles of the filter.

Note that any minuscule numerical approximation (which you are not free from) will alter the position of all roots. You will very probably get roots ending up outside of the unit circle (which makes your filter unstable).

So the ugly frequency response for the high order filter is expected.

As a rule of thumb, do not try to implement IIR filters of order above 10.