Function context scope doubt

Hi everybody,
Hope you all are doing great.

I have a question regarding how object's context works on functions at Node.JS.
For example:

function nestedFunctions() {
    var a = "I am a variable"
    console.log("Initial context:")
    console.log(this)

    function one() {
        console.log("First nested function, this is my this context:")
        console.log(this)
        console.log("First nested function, this is my a value:")
        console.log(a)

        function two() {
            console.log("Second nested function, this is my this context:")
            console.log(this)
            console.log("Second nested function, this is my a value:")
            console.log(a)
        }
        two()
    }
    one()
}
nestedFunctions()

The output of that code is:

Initial context:
Object Global
First nested function, this is my this context:
Object Global
First nested function, this is my a value:
I am a variable
Second nested function, this is my this context:
Object Global
Second nested function, this is my a value:
I am a variable

Everything OK so far, because the function context (this) is created on global scope, and so the nested functions. But my doubt is when running the following code:

class Dog {
    constructor(name) {
        this.name = name
    }

    bark() {
        console.log(`Woof, my name is ${this.name}`);
    }
    nestedFunctions() {
        var a = "I am a variable"
        console.log("Initial context:")
        console.log(this)


        function one() {
            console.log("First nested function, this is my this context:")
            console.log(this)
            console.log("First nested function, this is my a value:")
            console.log(a)

            function two() {
                console.log("Second nested function, this is my this context:")
                console.log(this)
                console.log("Second nested function, this is my a value:")
                console.log(a)
            }
            two()
        }
        one()
    }
}

let rex = new Dog('Rex')
    //rex.sitAndBark()
rex.nestedFunctions()

Output:

Initial context:
Dog { name: 'Rex' }
First nested function, this is my this context:
undefined
First nested function, this is my a value:
I am a variable
Second nested function, this is my this context:
undefined
Second nested function, this is my a value:
I am a variable

I am having troubles understanding that last one, and why the this of the nested functions is undefined. Why isn't the global context instead of undefined (like when declared outside a class, i.e, the first example)?

Best regards to all