"the second (II) does not and returns a NaN value"
Firstly, find never returns NaN. It can return an empty array, this is not the same thing at all as NaN.
I wouldn't be surprised that your second expression returns an empty array. numbers that are both smaller than a minimum while at the same time being larger than a maximum are ... very rare! You 2nd expression can be rewritten as:
II = find(y <= -90 & y >= 90);
which clearly is not going to be satisfied often.
In fact, only NaN numbers can satisfy this expression. So your find will return the indices of the NaN values in y or empty if there are no NaN.