It's not a trap. The Russians do not have the ability to provide food, fuel, and ammo for all of the forces that they have in the bridge head area. Nor can they provide food and fuel for the civilian population also trapped in the bridge head with them.
This is a partial explanation for why they did the civilian population transfer from the area, less civilians to feed = more food for the troops. And it explains why the Ukrainian efforts remain slow and steady.
