That practice is known as Block booking (https://en.wikipedia.org/wiki/Block_booking) and was supposedly ruled illegal in 1948. The more things change, I guess...
type Option<T> = T | undefined
function f<T>(value: T): Option<T> { ... }
let thing: string | undefined = undefined;
let result = f(thing);
Now imagine the definition of Option is in some library or other file and you don't realize how it works. You are thinking of the Option as its own structure and expect f to return Option<string | undefined>. But Option<string | undefined> = string | undefined | undefined = string | undefined = Option<string>.