The fact that they have just recently been able to form the cauldron lends some credence to this theory. Now, the Russians will have to push the sides of the cauldron, which will necessitate a much higher amount of troops. If they succeed, the payoff is immense - the 100k+ troops of the Eastern front will have to retreat without mechanisation, making a fighting retreat impossible. The Ukrainian army will be greatly struck, and the Russians will be able to take a lot of prime terrain and perhaps try a push all the way to the Dnieper and then up, completely cutting of Kyiv from any mechanised supply line.
Some have speculated that the only reason they attacked the North is to prevent the Ukrainian army from rotating their forced to the Donbass, thus maximizing the effectiveness of their small invading force. I don't know if I buy that.
In any case, it's abundantly clear the Russians are leaving the North to send the troops in the East, and are not being pushed back.