Japanese 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.

[1] https://0115765.com/archives/165655

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.