I think it is bad writing/setup if you have initially explained a setting and then contradict it immediately/with your initial characters. The big element here is the contradiction. When the setting is explained to the audience, they expect the land of fire elementalists to use fire magic. If the first character here isn't a fire elementalist and it is 1) not explained/not explained well and/or 2) it is not a main character where the difference is a key part of their identity/struggle it feels contradictory and confusing.
It is kind of ridiculous how often I have run into plots with background characters contradicting the stated setting for no discernable reason. Most recently it lead to one of the most frustrating and prolonged miscommunication plots I have seen (in an otherwise interesting concept in settings and characters).
I suspect in these cases the creators want their background or supporting characters to feel nuanced and they want to do it with contradiction like main characters usually get. However, during an intro this just makes the setting confusing. You do not understand why the characters are acting against the stated narrative elements, and no matter how interesting these supporting characters can be in the creators' heads they need proper development to pull this off. If this contradiction is not going to be the essence of the character, you just will not have the development to do it right unless it is a main proper.
I think most probably understand what I mean, but I think I still should alliterate it properly. So obviously in Harry Potter the three true mains are Harry, Ron, and Hermione, right? And the Weasleys are a bunch of supporting characters in general, right? Now imagine Jenny Weasley (the love interest of Harry that gets little to no development in the movies). Her family is known for being pure blooded, poor, red headed wizards in Gryffindor.
Now imagine the narrator says all that, but Harry meets Jenny as the first Weasley in the first movie; and she is a brunette rich girl in Slytherin. Then, because she is a minor character who isn't in the main plot until the 3rd movie, this goes unexplained or is dismissed with a summer job or that-one-random-relative-was-also-a-Slytherin-brunette claims. And then everyone moves on. Worse yet, even later on we find out there is no struggle or distinction here. No one acknowledges her contradictory appearance; she doesn't have a complex or struggle because of it; her parents and siblings and anyone else treats her like any other Weasley. Nothing about her contradiction is used to make her interesting, even after costing initial confusion. What was even the point of making her stand out if you don't develop it?
What do you think? This always frustrates me because it causes immediate dissonance when you are trying to understand and connect with a concept. I would consider this a genuine plot sin because it can easily kill possible connections. What is a plot pet peeve or plot sin to you?