It is during retreat that you can inflict the heaviest losses on withdrawing enemy
How exactly is it, that you know already in what shape the withdrawn/encircled Ukrainian formations are? From what is being reported by Russian war journalists, they took very heavy losses when withdrawing from the Gorskoye-Zolotoye pocket, and even heavier losses when swimming across the river from Severodonetsk to Lisichansk. Now the group trying to break out from Lisichanks to Seversk and Soledar are taking very heavy losses too, because the roads have been cut and all their escape routes are covered by artillery that's already dialed-in.
One Ukrainian unit commander posted on Facebook that it took him and his men 2 days of running between bushes, and crawling around at night through the tree lines to cross a few kms of terrain before reaching his lines. Again, Russian sources reporting that some withdrawing units took as much as 50% losses.
I think the gloves are off, and now the accent is on breaking the will of the AFU through a few very hot weeks/months.
Alternative explanation (if they really did pull out the majority of formations intact): it doesn't make a difference whether they are encircled now in a smaller pocket consisting of Lisichansk only, or a larger one that would materialize after Russian lines can connect through Slovyansk-Soledar-Artemovsk. Though I agree with you, the Russian army should be raining constant hellfire on them during their retreats.
Finally - the Ukrainian explanation. They have fooled us. Through disinformation in media, they convinced us they were still holding on to Severodonetsk, when in fact for two days they were quietly retreating in small numbers and mining key axis of movement. A small group of rearguard forces continued to make noise to give us the impression of more serious numbers. With their men safe, they continued to calmly watch us expends tens of tons of ammunition daily on empty positions. Finally, a stupid Ukrainian journalist blew the brilliant operation by announcing that Ukrainian soldiers were successfully withdrawn. This triggered the movement of our forces into the city, and the end of this chapter.
You are a smart man, you decide which scenario is plausible.