When I was doing a lot of sports (though nothing quite as high contact as grappling), I'd put a couple of strips of kinesiology tape over the sensor which helped a lot with keeping them on. You need to put something non adhesive between the back of the sensor and the tape though, to avoid pulling it off when you change the tape.

I don't think it'd stand up to bjj I've dislodged one or two of mine (over three years) by knocking or catching it on things without great force.

For non-contact sports (football) mine have been fine though.