I'm also finding other words as I go through the review process. "reinfrocement" wasn't flagged. So I just tested a bunch of words by switching one letter in the word -- word doesn't pick any of these random words up, even though they're all misspelled. Refrinement minmize ancohrage calclulation

I thought there was a setting somewhere that auto-corrected swapped letters, but I might be mistaken; and it's not working in any case

But with or without that swapped-letter setting, shouldn't Word pick these up anyways? With the swapped letter, they still aren't real words...

I can't speak to the spellcheck part of the problem, but may be able to help with the autocorrect.

Swapped letter correction is handled via autocorrect feature; there must be an entry for that specific misspelling--it doesn't look like the default Word autocorrect list has any of the ones you listed.

If those are typos you commit often, you can add to the autocorrect list (File>Options>Proofing>AutoCorrect Options)

Solved

As Charles hinted, the problem had to do with the "do not check spelling and grammar" setting. It was UNSELECTED, which implies it should be checking. Nonetheless, I changed the language from English (UK) to English (Canada), closed the dialogue and it was checking spelling! Then I went back into the dialogue and noticed the checkbox was SELECTED -- this didn't make any sense... I switched back to English (UK), Selected and unselected the dialogue box, and did all this a few more times and it settled back into a correctly working system. So I can only partially explain what happened there, unfortunately; but it's working for me, fortunately.