This is because as is a valid place for contextual type inference. Just change as to satisfies and you will preserve the function return type.

But then it's not an unnecessary type assertion either. I mean you can use the type assertion satisfies is just better.

I mean the error is a little bit misleading as the quick fix suggests removing the type assertion not refactoring it to satisfies. In my case the type assertion / or the satisfies (either of them) is necessary, not optional.

Either the error should be removed or the quick fix updated I suggest.

Alternatively you can use as const for this.

Here is a more minimal example:
let a = (Date.now() % 2 ? 'a' : 'b') as 'a' | 'b';

Noting that this is closely related to #8721

Reopening since we reverted #10523.

I have a similar bug, and I'm not sure if it's the same as this one or a different one because, differently from the one in this PR, changing as to satisfies as suggested above (#10257 (comment)) changes the types of the code

https://typescript-eslint.io/play/#ts=5.5.2&fileType=.tsx&code=GYVwdgxgLglg9mABMMAKA7gLkQIgIY6IA%2BuARjgJSIDeAUIohAgM5SIAOeUUApgE5IAvIgAGAEmroAviMR5moiVACe7HnGCJpIgNy16iPjyggBHLrwF6p%2BgPS3EASU3K4IQzwC2cAG49EUAAW-jjgYDwQPMzMeHzKctH8sAiEeKS%2BPAA0AYEwCvx8cHzMtAA2xh7M2PiEJDjkiMIoqDUUekA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQAIBcBPABxQGNoBLY-AWhXkoDt8B6Jge1tiacTJTIAhtEK0ipWkOTJE0fJQ5N0UOdA7RI4MAF8QOoA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

function fn(w: "a" | "b") {
  const pattern = `${w}` as `${typeof w}`;

  return pattern;
}

// If you remove the "unnecessary assertion" above, this errors
let res: "a" | "b" = fn("a");

EDIT: This is exactly the same as the one in this issue, and replacing as with satisfies indeed works. I don't know what I tested 😅