Goofy `instanceof` refinement under a bounded polymorphic function

I've got a polymorphic function taking two unrelated classes, and I'm trying to refine an union based on its inputs, e.g.

```js
class X {}
class Y {}

function testOuter(_A: Class, _B: Class): void {
declare var value: A | B;
if (value instanceof _A) {
/* All of these pass */
//(value: A);
//(value: B);
//(value: A|B);
//(value: A&B);
} else {
/* Only `A | B` passes */
//(value: A);
(value: B);
//(value: A|B); // Among these 4, only this passes
//(value: A&B);
}
}
```

I was expecting `(value: B)` to pass, but it doesn't. Further, the consequent branch passes `A&B` (and any other permutations of `A` and `B` under union and intersection).