Why do AI detectors flag non-native English writers more often?

My situation: I am a graduate student in civil engineering and English is not my first language. A report I wrote last term was flagged by a detector, and I had to explain myself to a supervisor. It was resolved without a penalty, because I could show my notes and the history of the file. But it was an unpleasant week, and I now write with the feeling that my ordinary style is under suspicion.

What I have done since:

• I turned on version history for every document and keep my handwritten outlines
• I read the paper by Liang and colleagues on detector bias, which tested several detectors on writing by native and non-native speakers
• I stopped asking AI tools to "improve" my drafts, because I could not tell whether that made the flag more or less likely

What I do not understand yet:

1. Why exactly does simple, correct English look "AI-like" to these tools?
2. Is making my sentences more elaborate a sensible response, or does that just make the writing worse?
3. What can I give a supervisor or a university panel, so the conversation starts from evidence and not from the score?

I would appreciate answers from people who have been flagged, or who handle flags.