We never hear about Ukrains retreats - most people know the ground fighting consists of advance and retreat many times over for both sides - thus the nature of war.
The Ukrainians have had years to fortify eastern Ukraine. My guess is some of the battles resemble small scale versions of Soviet assaults on dug-in German positions in WWII or even WWI trenches. Murderous for both sides, but the Russians are probably losing the usual 3-4:1 for offense vs. defense casualties. Only unlike WWII, they don't have the reserves to make up those losses.