keyof operator behaves differently when used as a generic

Question

I've bumped into a situation when the usage of the keyof has a strange behavior.

Let's assume I have the next interface:

interface Data {
    someKey1: string,
    someKey2: number,
    someKey3: boolean
}

And I want to iterate over keys of an object which implements the interface and assign a value to a separate object.

    const data: Data = {...}; // it has the data inside
    const dataCopy: Partial<Data> = {};

    Object.keys(data).forEach((key: keyof Data) => {
      dataCopy[key] = this.data[key];
    });

The ts linter shows me the next error:

dataCopy[key] = this.data[key];
^
Type 'string | number | boolean' is not assignable to type 'never'.

But, if I will use the same logic but with generic, it will work.

Object.keys(data).forEach(<K extends keyof Data>(key: K) => {
  dataCopy[key] = this.data[key];
});

Question

Why is it actually happening? For me keyof K and <K extends keyof Data> are describing the same thing and should work the same in this case. Could anyone explain the behavior?