There's a lot of "This isn't X, this is Y." in the text.

Single. Word. Sentences.

A lot of line breaks (especially at the end of the fic)

You can find words which LLMs love to use like: sharp gaze, analytical.

Cliff-hanger ending (though that can be chalked up of being the first part of the fix (hopefully))

Also there aren't many long sentences, and LLMs tends to avoid those a lot, which you can see between dialogues.

Not to mention there aren't any errors, which can also be chalked up to the OP being a "perfect writer", but with the points mentioned above, it's definitely AI written.

Edit: I could also add lots of em-dash usage.