"arguments" variable in JavaScript arrow functions not debuggable?

I was reading this on MDN - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

In a section titled "No binding of arguments" I saw the following and tried it on the browser console. Works.

No binding of arguments

Arrow functions do not have their own arguments object. Thus, in this example, > arguments is simply a reference to the the arguments of the enclosing scope:

When I tried to debug the arrow function, the value of arguments is different and unexpected. Is this a bug in DevTools or am I overlooking something?

In the example below, arguments.length printed in the console is 1. (The value being 1)

However, when execution stops at a breakpoint, arguments.length is 4. (The values being 1, 6, 7, 8)

(function() {

    var obj = {
        fn: function(a) {
            var f = v=> { 
                debugger; 
                console.log("Arguments length " + arguments.length);
            };
            return f(a, 6, 7, 8);
        }
    };
    obj.fn(1);
}
)();

jsFiddle here - https://jsfiddle.net/3Lb640aq/