I can assure you it does work. I've been doing it consistently since Grievous was launched back in October 2018.
The issue here is probably you trying to hit him in the back, but somehow it not registering that hit. That may be due to lag, to hit delay, to hit detection issues, to deceiving angles, and more.
But in a controlled environment, or if the game processes it right, you can attack grievous in the back while doing Unrelenting Advance and it'll work. I kill grievous players like this all the time. It seems harder to pull off when he's rotating in place, but if he starts chasing an ally of mine, then I can lend some 3 or 4 swings on his back and that means he's probably dead.
