Why TS raise no index signature found error in one scenario but not the other one?

Question

I think I know the reason this error is showing but the thing I don't understand is why Typescript feels the second scenario is safe and therefore doesn't raise any error?

interface TagsType {
  tag1: string;
  tag2: string;
}

let tags : TagsType = {
  tag1: "some tag 1",
  tag2: "some tag 2",
}

Object.entries(tags).forEach(([tagName, value]) => {
  /*
  Element implicitly has an 'any' type because 
  expression of type 'string' can't be used to index type 'TagsType'.
  No index signature with a parameter of type 'string' was found on type 'TagsType'
  */
  tags[tagName] = value;
  // Fine
  tags = { ...tags, [tagName]: value };
});