Check this comment Refusal of UK Standard Visitor visa under v4.2(a)(c), v4.2 (e) and v4.3(a)(c) and note that the user has been suspended from chat.
You can try to invite her to chat and you'll see the red flag. There are no apparent signs of misbehaviour. She has cleared the 20 point rep hurdle.
Is it a bug? Can one of the higher-ups have a look-see?