In Arceus and The Jewel of Life, we see the fight between Arceus vs Giratina, Palkia, and Dialga. Arceus was missing the types of Water, Electric, Ground, Dragon, and Grass. So why did Palkia, Dialga, and Giratina's Dragon moves not work on Arceus?
I wouldn't call it an error, either the moves you thought wre dragon moves weren't or he wasnt immune but he became a type that resists dragon like steel; and arceus, being arceus, took almost no damage.