Allow me to repeat myself:
I am not aware of any precedent when it comes to pixel art, but in music, landmark US cases have upheld copyright for musical motifs consisting of as little as four (that is the number that comes after three) notes because even that constitutes enough creative effort (the actual standard that is being applied) to constitute copyrightable material.
I would also argue that, if making pixel art doesn't take enough effort to justify copyright, surely you can just make your own without using AI. Right? You'd have to be a pretty big idiot if you need AI to generate the artistic equivalent of a single musical note.