This is not a case of precedence, but of evaluating function arguments.operator is a function with one argument, and this argument must always be evaluated before passing it to the function. Since in this case the argument is an expression, the result of the expression is used as an argument. You could use the postfix increment instead if you wanted to pass 0, rather than 1.
The operator precedence rules would only be applied if you used the increment operator on
int num = ++buff[idx];
P.S. - an explanation of my first statement:
In C++ you can overload definitions for operators working on arguments of a specific class. This is called operator overloading. There are plenty of articles on this topic, e.g. here
], but the one thing I was referring to is that when you overload the  operator, the signature of the function must look like that of a function with one argument: the index value of the element you want to access.
That means whenever you invoke the  operator, it is internally treated like a call to a function, and the first thing that is done then is that it's argument, the index value, is evaluated.