This is a bit of a compromise fix - when we see a property that came from a mapped type (or reverse mapped type) instantiation where the input and output are optional, if the type contains an actual undefined (and not missing) type, we now always write that | undefined to the output. It's not strictly required outside of exactOptionalPropertyTypes (since the optional prop and the optional prop with an | undefined are functionally identical), but keeps the output consistent between the two modes. More relevantly, under exactOptionalPropertyTypes, we only add the | undefined to the preserved node if there's a real, explicit undefined in the type. By limiting this to only the results of mapped type instantiations, I think I make it so this has no impact on isolatedDeclarations behaviors (since those can't resolve a mapped type).

I think we want this?

@typescript-bot cherry-pick this to release-5.5

Starting jobs; this comment will be updated as builds start and complete.

Hey, @jakebailey! I've created #59230 for you. This involved updating baselines; please check the diff.