I swim daily, not the same impact as BJJ, but use a Libre armband designed to protect the sensor and keep it in place, not sure if it’s got enough stretch in it on arm for BJJ arm movements in muscle itself. But I have noticed others in the pool with sensors…one woman wore hers on the front of her shoulder probably below the clavicle inside bra strap height….might be displaced as gi pulled! Another man in pool wore his much higher on top of shoulder. I have heard of others who wear them on front of hips, sort of in area one injects insulin. Perhaps you could trial the position by using an old sensor ( with pin chopped off) held in place by methods suggested here, rather than a real one, to see which stays in place and is comfortable…or swop sparing partners to one who favours a different position!