Spellcheck doesn't see the whole work when suggesting fixes if you've tried to fix the spelling on your own by editing the word. It properly identifies if the work is right or wrong, but the suggestion only sees the letters before the edited content when deciding what words with be correct.
To recreate type or paste "intarmediate intermediate" into a chat. Replace the first e in the second word with an a so it matches the first word.
Right clicking on the first word offers the correct spelling. Right clicking on the second word offers "anti" as the first choice and "intermediate" is not offered.
Spell check will recognize that you spelled it right if you fix it manually.
You can also break the spell check on the first word by bolding the first a.