This happened in the Iraq war. It wasn't safe for Western forces to patrol freely so they ended up behind heavily fortified bases. Put yourself in the shoes of a Russian in a forrest trench at night. If you see Ukrainians passing through are you going to start a fight, give away your location? What would you gain? The difference between the U.S. in Iraq and Russian in Ukraine is Ukraine has the weapons to attack those bases. Serious weapons, not the small stuff Iran smuggled into Iraq.

An irony is if Russia left the cities alone and just built bases away from them it could have forced Ukraine into classic warfare. Instead, Russia bombed the cities and and now makes it easy for Ukrainian partisans to attack because they don't have to worry anymore about getting someone's home destroyed by return fire. Russia is caught between the proverbial rock and a hard place.

If they build bases they will fall to Ukrainian artillery and rockets. If they spread out they will be picked off by partisans.