Back in 2019 I booked the cheapest advertised economy fare from MCT-DOH-KTM. I saw a stack of boarding passes to be handed out at the gate for the DOH-KTM leg and was handed one for an upgraded seat in J. I was only a BA Silver at the time. I don't think I've ever been upgraded on BA as a Gold even. Perhaps the chance depends on the route and how many other high-status FF on a particular flight?