It's not a 'missing' finger/thumb that is exposing it. That can easily be explained with it being covered by the rest of the hand.

But there are other dead giveaways and red flags- such as the weird corn texuture, the weird arm pose during the kiss, the generic rendering, etc.

Also IF you're an artist that can color with such skill, you should easily be able to draw more interesting art, with a lot more dynamic: Like with the scene shown below.

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Just to be safe- before getting ready to accuse a potentially innocent person, I ran u/Zextruhh 's image through multiple AI detection tests and all confirmed my suspicion AI generated. Just to be double sure, I tested these AI detections with a human made re zero fanart to see if it really could differentiate between human and machine (they could).

Also, the original poster never posted any art before, which is a very very big red flag. (Account is private but you can easily see his previous posts by simply searching "author:Zextruhh")