Russia is working from fear outward from Putin. Russians probably except Wagner, but contract, conscript, convict, snatched up Ukrainians, and all including North Koreans soon stay put or move forward mainly out of fear of being shot by the guys behind them. If at some point the guys in front are more scary and dangerous than their own ranks behind, they break and break extra disorderly.
Ukrainians have a reasonable expectation not to be deliberately shot by other Ukrainian soldiers. This turns out to be a huge advantage when it comes to holding ground.