About the Seiji unfinished bust-up, based on a similar-looking unused portrait for Yui, it seems like PWG/Black Wings is most likely using AI for at the very least these extended portraits. :( It's a shame to see, honestly.
First and foremost, I went back and found the Colorful Yui base that you guys mentioned; there were unused bases for Violet, Fuuka, and Soy there as well, though not as extended as the proper full-size portraits (and these four all have a slightly different file name convention, which is why I didn't notice them sooner):
Now that you mention AI, and now that I see these, I'll admit, the buttons on concept Seiji, Violet, and Yui's clothes are pretty strange, in a very suspicious way. If an artist was making a quick concept mockup, they'd probably make the buttons match up more with the existing ones (or draw over them entirely), and whatever's going on with the buttons on the concept Seiji's overshirt is way beyond that.
I would guess that a human still drew the final version of Seiji's sprites, like the second anon suggested, because the buttons (and details in general) are a lot more cohesive there. But it's absolutely a shame if they're using AI in any part of their process, even if it at least doesn't seem to be making it into the final game at the moment.
