Grievous doesn't take damage during Unrelenting Advance

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.

Yes I can assure you it's about the only one of his abilities that you can deal a lot of damage and even kill him in unrelenting advance if you get behind him. In fact this ability is way weaker than it used to be and without his o.p. crawl rush he would be a mid tier character with all his bugs IMO. I do notice however the hit registration for this game has got way worse and I hear it's due to the servers being downgraded which I can believe since I've personally being getting worse input lag since they stopped supporting the game with patches (well the bigger ones, not sure if they'll do smaller fixes) so I think you're just getting no hit registration caused by the server.