Parece não ter efeito algum quando eu digito palavras incorretas.
A explicação mais provável é que o Notepad não usa a API do Windows Spell Checking.
It works in some universal apps (especially built-in apps like Edge and popular apps like Facebook), but not in others. It’s up to app developers whether they want to use the Windows Spell Checking API or not. Autocorrect doesn’t work on most traditional desktop apps at all. It can even interfere with other spellchecker apps or services you might have installed, such as Spell Check Anywhere or Grammarly.
(ênfase minha)