The Ukrainian military hasn't won a single battle against the Russian military nor is it possible that could.
” places Russians did want to defend”
Russia retreated from Kherson right after it had declared it had annexed Kherson. Sounds like it couldn’t defend that bridgehead as much as it would have wanted to do so.
The Izyum-Sumy area was excellent defensive terrain. Russia also lost the direct rail line from Belgorod to the Donbas, its principal logistics route.
That is, Russia has a potential incapacity in defense, and is likely to be vulnerable to losing ground.