I honestly believe even some writers buy into the misconception that Maul was just a "pawn" or "tool" rather than a real apprentice/Sith Lord, which furthers the misconception, e.g Sidious' mention in the Book of Sith. All Canon sources, Darth Plaguies, the Dark Side sourcebook and a few others have the correct interpretation of Maul's standing, even if the latter two are outdated in some ways.