How did the Soviet Union government treat Red Army soldiers who had surrendered on the Eastern Front ?

First the context of Order 270, requiring each soldier to "selflessly fight to the last", needs to be examined. In 1941 desertion and insubordination were severe problems in the Red Army. Soldiers surrendered without a fight or mutilated themselves to avoid fighting, and worst of all officers and commissars acted to save themselves rather than lead. While most soldiers of the Red Army fought bravely the situation was bad enough that Stalin, not averse to utilizing terror, made clear what the punishment would be for giving up without a fight. Of course for most soldiers on the ground Order 270 made little difference, but it was of symbolic importance.

Fast forward to 1944-45, Red Army began liberating POWs in large numbers. Now in the interim years the NKVD, in charge of everything from border security to firefighting, gained experience in processing liberated prisoners and civilians. More importantly its goal was now to punish those whom had collaborated with the Nazis, civilian or military. For instance men who agreed to work for the Nazis as Hiwis (Helpers) doing jobs such as carrying ammunition would be imprisoned, executed, or sent to penal battalions once recaptured.

The majority of liberated prisoners would be sent to filtration camps where they were interrogated and investigated by Smersh (Smert Shpionam, "Death to Spies"). As said above, the goal of this was to sort out those whom had collaborated with the Germans, deserted, or had other questionable circumstances surrounding their capture and imprisonment. Here's an interview with Gregory Davidovich Vodyansky about his experience in a filtration camp:

Q: How was the filtration conducted in the Murom filtration camp?

A: We were lodged in the barracks on the empty wooden plank beds.

There was no physical punishments or such applied to us. But all the time we could hear threats from the guards and the investigators. During the first days the “suspicious” individuals were separated, as well as the officers from the ranks.

The investigators were calling in people one by one for a thorough interrogation. After about a week my turn came. The first thing I heard from the investigator was: “Are you Grigory D. Gurin? Take a sit, traitor! Tell us where and when and how you surrendered to the enemy?” I replied: “Well, I am OK standing. And my surname isn’t Gurin but Vodiansky”. His reaction was promt. He jumped up and said right into my face: “Are you implying that you are a Jew? Then tell me how you, a Jew, managed to survive in a German concentration camp?” Then I presented my detailed account naming all the units I was serving in RKKA, objectives our Airborn Brigade, the circumstances of my surrender (I was injured) and that there are two alive witnesses to it.

When I ended my narration, the investigator was silent for a while and then said: “Well, you dismissed for now… For now… Expect the next interrogation session.” After two weeks the guards called me in again. Unlike the first session he was friendly. He offered me a chair. Then he asked me strictly: “Why did not you let your parents know that you are back and healthy?” I said that I did not know their whereabouts. They were evacuated to Cheliabinsk, but it was two years ago. The investigator replied: “We pulled some strings and found out that your parents live now in Ukraine in the town of Herson.” He gave me the address and told me to get in touch with them. At the end of our talk he said: “Expect to be called in again, but next time it will by other people.” Yes, next time I was called in by people who arranged the job placement fro the ex-POWs in the civilian sector. I received the temporary ID card, whish stated that I passed the filtration and is cleared, that I am a Soviet Union’s citizen and have right to vote.

I was sent to town Rostov to work on the limbering enterprise. On the 10th of June 1946 I left that work place and headed to town of Herson as a free man. But after arriving and as soon as I registered I was again called for a talk to the local State Security Department office. These continued for several months. The common civil passport I received only after 6 months.

All POWs were considered potential traitors and treated as such. Smersh, with years of war experience under its belt and motivated by Stalin's desire to repress potential threats to his rule, was very effective at hunting down and sentencing suspicious individuals. The vast majority of POWs were released but lived under constant threat of further investigation to the point where trying to meet with former friends was dangerous.

In total out of 1.5 million POWs processed in filtration camps 15% were sent to the NKVD, 22% were sent to labor battalions of the Ministry of Defense for infrastructure work for 1-2 years, 42% were conscripted into the regular army, and 18% were sent home.

POWs were treated very poorly by the Stalinist system during and after the war. Even after they were initially cleared they suffered through years of institutionalized discrimination. But it is a myth that all POWs were handed over to the NKVD upon liberation and imprisoned for decades; generally there was another reason behind it beyond merely having been captured.