why typescript behaves differently when using the type directly vs using generics with extends?

I dont understand why the code behaves differently when using generics.
The topic is based on the answer for How to create a relation between parameters of a function?

interface Data {
  a: number,
  b: { x: number, y: number}
}

type GetKeyValueTuples<T> = { [Key in keyof T]: [Key, T[Key]] }[keyof T];

function getChangedDataByProperty<T extends Data>(
  ...[data, changedProp, newPropValue]: [T, ...GetKeyValueTuples<T>]
) {
  if (changedProp === "b") {
    changedProp
    return {
      ...data,
      b: {
        // why this has number, a, b types?
        x: newPropValue.x,
        y: newPropValue.y,
      }
    }
  } else {
    return {
      ...data,
      x: newPropValue
    }
  }
}

// why this behaves differently than the abvove function?
function getChangedDataByProperty2(
  ...[data, changedProp, newPropValue]: [Data, ...GetKeyValueTuples<Data>]
) {
  if (changedProp === "b") {
    changedProp
    return {
      ...data,
      b: {
        x: newPropValue.x,
        y: newPropValue.y,
      }
    }
  } else {
    return {
      ...data,
      x: newPropValue
    }
  }
}

The ts playground