alt Hacker NewsJapanese isekai might not be your cup of tea, but one example is [1] which was way back in Jan, and models have improved a lot since then.
My sneaking suspicion is that there are a lot more cases than get reported, because authors are disincentivized to disclose AI usage, and readers by and large feel icky about reading AI work for reasons unrelated to quality.
I can totally see a weird future where a lot of what people like is AI generated, and everyone lies to each other about it. So it's hard to do research on this phenomenon.
Not completely surprising that AI could do well generating manga. It's very formulaic in general and by nature devoid of a lot of the relatable character development from most other writing. I have no problem reading an AI story if it's done well, but it typically just isn't. What I find annoying though is AI generated content that is only interesting if it's actually real. Like a historic photo or a video of some shocking event.