After reading this thread and found out they made other games (very few of them). I looked up their pixiv fanbox and some of the rewards are speedpaint videos. I am no artist and I have not seen their speedpaint vods, but just the fact they are offering them as rewards is proof enough for me to believe they don't use ai. I still could be completely wrong.
I could be wrong as well. There is just this nagging feeling, like when something just doesn't feel right.
Looking at their DeviantArt and pixiv it is for certain, that they published their own art. At least that is my conclusion when looking at their earliest posted work (around 2016). Of course over time art styles change. And glaring mistakes by AI can be hidden by someone, who knows how to do it and cares enough about doing it.
An example from 2020 on their DeviantArt site, so before the big boom in AI art. The shine in the eyes. The smooth change in lighting. Chef's kiss.
If I had to point at one piece between 2020 and 2025 to be certain of a change, I am choosing this from March 2022 on Deviant Art. It is the overdone shine on the hair, the glaring speck of light on the nose and the hard border between light and shadow on the fingers for me
Now let us look at 3 pieces from the pixiv fanbox. I don't know how to tell you this, but I have seen enough AI pictures on f95zone alone, to see, that this is AI generated. The lighting. The smiles. It has something uncanny to it.
Without seeing the speedpaint videos I can't say anything about them. Relatively cohesive AI videos of an art process are possible. At least I conclude that from a quick internet search on the topic of a tool called Sora.
I downloaded the game and it is only short animation loops. Interaction only through buttons. Miyabi's nipples have different hues in it as well even if taking shading into account.
For me my current verdict is, that WaterRing has become an AI artist over time.