Your suspicion about why you got flagged is well documented. A widely cited Stanford study found detectors flagged the majority of essays by non-native English speakers as generated while almost never flagging native speakers, because simpler vocabulary and more predictable sentence structure is exactly what the statistics read as machine.
That is the reason several universities stopped relying on detection, and why most current guidance for teachers says a score should start a conversation rather than settle one. Quote that at them.