A score that checks a draft against a record can only be as right as the record. If the record holds an error and the draft copies it, the draft passes, because it matches.
The sentence
A note about three assistant personas said that two of their three names came from one novel series, and that “the third is an animal”. That was false. All three names come from the same series, and none of them refers to an animal.
Where it spread
A writer drafting from the note was sent back to that sentence during a revision, and wrote that the third name “belonged to an animal”. Two scorers checked the draft against the note, and both passed it. The draft agreed with the note, and the note was the thing that was wrong, so neither scorer had anything to catch.
What caught it
A person read the draft and knew what the three names were.
The note was corrected with the false sentence quoted before the fix.