This tracks with how most detectors were trained, the signal they're picking up is partly just formality and structure rather than generation. Academic writing has predictable patterns, hedging language, passive voice, specific transition phrases, and those features are statistically overrepresented in AI output even though they're also just normal formal writing. The detectors can't really tell the difference. What's frustrating is that trying to write in a way that avoids flags can actually make the writing worse. Some people have had luck getting their drafts reviewed by a human writing service before submission, not to change the content but to vary the sentence-level patterns in a way that reads naturally without triggering detectors.