there's a lot of sunlight between the class of things that are idiomatic and the class of things that are anti-patterns; and there are likely things that are idiomatic but still anti-patterns (yes, you can do this, and if you did this it would look like this, and it causes no regression in this particular case, but don't get in the habit of doing it this way because it can cause a hard-to-spot regression in the general case)
To you. English has a variety of suffixes that allow for exactly this kind of constructive generation of new words. Thus 'patternic' is a word precisely because it has just been coined. Or do you only ever approve of "dictionary" words without pondering where they might come from?
it's an idiotic construction— the prefix may be originally greek, but has taken a specific connotation in english, while 'pattern' is from french, and the '-y' suffix is more common in english anyway, so the correct form for the idea would of course be anti-pattern–y