BS, never happened, the reason they would was they were trying to win. Nearly all claims of match fixing are BS especially in light volume fixtures like the LOI. I met friend outside PP one day clutching a roll of money, he had tried to get a substantial amount of money on an outcome involving Sligo Rovers in Europe, they offered him 5% of his stake at half the price.
The problem is getting on and getting paid, of you do manage by timing and subterfuge in getting laid in a shop if the layer suspects fixing they will downright refuse to pay and there is nothing you can do about it. Bookmakers software is so sophisticated that it can detect punters that are canny and don't lose enough money and restrict them without a trader even looking at their account.
If you use an exchange any suspect betting patterns will also lead to suspension of the market/payment and the amounts available on either side of a LOI game are so thin that 200e could shift an outcome from 2s to levels and that is an outright not a derivative, the derivs are normally so thin and overound to be not worth bothering about anyway.