When they were releasing the official artwork for the new Pokémon throughout the year, they started releasing them only with a white outline, and then started releasing them without. We had no choice but to leave this inconsistency until release because it's all we had, but now we have the actual game assets for every new Pokémon so they are at least consistent there.
Once we get hold of the official art for all of the new Pokémon we'll update them all at once with high-quality, consistent images. Same goes for the HOME renders once those are available.