I have seen others complain about the way this works and I agree with them.
When you mistype a word (I never misspell words) it is underlined in red and a correction is usually proposed, so far so good. The problem is that you have to actually reject the suggested correction, if you just press space, it is adopted.
This is fine if the suggested correction is the intended word otherwise quite annoying. For example, just a few minutes ago, "fragile" got replaced by "erectile" in a post that I was typing, amusing but also a bit of a nuisance.
As far as I know most other spelling checkers leave mistyped words unchanged unless you explicitly accept the suggested correction.