Your MyUnionType is defining an array (of any length) that only contains 1s and 2s. Setting values to [1,2] doesn't narrow the type down to a tuple of 2. I'm not sure why you think it would, but feel free to explain your reasoning and maybe I can clarify your misunderstanding.

In any case, it seems like you might be looking to use tuple types

// this is an array that can contain 1s and 2s
type ArrayOf1or2 = (1 | 2)[];
  

// fine, because regardless of the length, it only contains 1s and 2s
const ok1: ArrayOf1or2 = [1,1,1,2,2,2,1,2];

// no good, because it can't have a 3
const notok1: ArrayOf1or2 = [3];

type Size2ArrayOf1sAnd2s = [(1 | 2), (1 | 2)];

// has 2 so is fine
const ok2: Size2ArrayOf1sAnd2s = [1,1];

// has 1 so is not fine
const notok2: Size2ArrayOf1sAnd2s = [1];

// has 3 so is not fine
const notok3: Size2ArrayOf1sAnd2s = [1,1,1];