Hey everyone could somebody help me to understand the following situation:

export function functionThatReturnsTurpleObjects():
  | [{ a: number }, undefined]
  | [undefined, { a: number }]
 {
  if (Math.random() > 0.5) {
    return [undefined, { a: 1 }];
  }
  return [{ a: 1 }, undefined];
}

function execute1() {
  const [res1, res2] = functionThatReturnsTurpleObjects();
  if (res2) {
    return res2;
  }
  // res1 can't be undefined and ts understand it
  res1.a;
}

function functionThatReturnsTurpleObjectAndString():
  | [{ a: number }, undefined]
  | [undefined, string] {
  if (Math.random() > 0.5) {
    return [undefined, "1"];
  }
  return [{ a: 1 }, undefined];
}

function execute2() {
  const [res1, res2] = functionThatReturnsTurpleObjectAndString();
  if (res2) {
    return res2;
  }
  // res1 can't be undefined but ts throws error "TS18048: res1 is possibly undefined"
  res1.a;
}

link to playground

I would expect that both cases should work in the same way.