Soviet POWs returning from German captivity were subject to political repression, although there is some controversy over how common this repression was and the numbers involved. Russian sources, working from Soviet archival records, have generally claimed that the majority of repatriated POWs were quickly released or reintegrated into the Red Army and only a minority were sent to the Gulag, while some Western historians believe the Soviet records are inaccurate and the Soviet government's official histories were purposefully altered to hide the repression of POWs, causing subsequent works in Russian to be inaccurate. I'll try to give you an idea of the known facts as well as the historiographic issues at play.

The situation of returning POWs was problematic in light of Order No. 270, issued by Stalin on 16 August 1941, which forbade Soviet officers from surrendering to the Germans and required them to fight to the death if encircled. Under this order, officers and soldiers who surrendered were considered deserters who were subject to execution if caught attempting to surrender. Stalin idealized commanders like Ivan Boldin, whose unit had fought back from behind enemy lines after being encircled during the Battle of Bialystok-Minsk in the opening days of the war; by contrast, his superior, Dmitry Pavlov, the commander of the Western Front, was executed as punishment for his forces' defeat. By the time the order was issued, hundreds of thousands of Red Army soldiers had already been captured by the Germans, and by the end of 1941, more than 2.3 million Soviet soldiers were in German captivity, the vast majority of whom did not survive until the end of the war. A total of 5.7 million Soviet POWs ended up in German captivity during the war, of whom 3.3 million (58%) died, mainly due to starvation and disease.

Beginning in late 1941, the NKVD established so-called "filtration camps" for the screening of returning POWs, who were interrogated and, in some cases, placed on trial for desertion or collaboration. Under Order No. 270, the repatriated officers were stripped of rank and imprisoned or sent to serve in the penal battalions (shtrafbats), which were established in July 1942 under Order No. 227 (the famous "not one step back" order). According to the main Russian source on repatriation, Viktor Zemskov, the majority of prisoners repatriated during the war (approximately 90%) were cleared of charges of desertion and returned to service, while only 8% were imprisoned.

The flow of returning Soviet POWs increased greatly in 1944 and 1945 as the Red Army liberated German POW camps, along with large numbers of Soviet civilians who had been conscripted as forced laborers (Ostarbeiter). The Soviets tried to streamline the system of filtration to decrease the backlog, and opened more than 100 new filtration camps for this purpose; however, the bulk of the process of repatriating former POWs was not completed until 1946, and smaller numbers continued to be repatriated and filtered until the early 1950s. According to the official Soviet archival records, which are referenced in the works of Zemskov and another Russian military historian, Grigorii Krivosheev, about half of the repatriated prisoners were returned to active service (for men who were still in the age range subject to the draft), 20% were released (for those older than the draft age), 20% were sent to forced labor (in penal labor units or industry), and the remainder, which varies from about 12% (Krivosheev) to 15% (Zemskov) depending on the source, were sent to the Gulag.

The question of how many of those sentenced to the Gulag were genuine collaborators and how many were subject to a purely political repression is still controversial. We know that thousands of Soviet POWs did collaborate with the Germans, either serving in one of the collaborationist military formations that fought against the Soviets (such as the Russian Liberation Army, led by General Andrey Vlasov, with over 100,000 men) or working for the SS as guards in the concentration camp system (the so-called Hiwis or Trawniki men, named for the camp where they were trained, of whom there were about 5,000). Of course, the use of the term "collaboration" here is complicated, because for many of these prisoners, the choice was collaborate or die, due to the horrific conditions in the German prisoner of war camps. As I noted above, 58% of Soviet POWs in German captivity died. As a result, the question of Soviet POW collaborators continues to be complex, as demonstrated by the trials of accused collaborators like John Demjanjuk. We know that many of these alleged collaborators (like Demjanjuk) managed to escape to the West through the displaced persons camp system by misrepresenting their identities and their actions during the war. It's probably not possible to determine from the existing records how many of those convicted of collaboration were genuine collaborators and how many of them were falsely convicted, whether accidentally or deliberately.

As I noted above, the figures presented by Russian authors like Krivosheyev and Zemskov have been disputed by Western historians. I admittedly haven't gone through the Soviet documents myself, since I haven't gotten that far in my own research on Soviet POWs, so I can only report on the historiographic questions second-hand. Some Western historians of the Soviet Union have indicated that they believe the Soviet figures and the histories based on them are trustworthy, but other historians have accused the Soviet authorities of suppressing information about prisoners of war, suggesting that they believe the figures on which the Russian studies are based are inaccurate or falsified. Soviet dissidents like Alexander Solzhenitsyn have also claimed the repression of POWs was more widespread than the official statistics acknowledge. As I said before, it's probably not possible to reconstruct an indisputably accurate version of those statistics, since we don't have the means to verify which of the Soviet convictions were legitimate and which ones weren't, or even the means to fully authenticate the numbers those statistics present; indeed, even Zemskov and Krivosheev don't agree precisely on the numbers. It's a subject that needs more research, both re-evaluating the existing source material and attempting to find new material that can verify or dispute the existing records.

Regardless of whether the official figures are accurate, we know that hundreds of thousands of returning Soviet POWs were subject to some form of political repression, whether it was in the form of penal labor service or imprisonment in the Gulag. Many of those who were imprisoned were not released until after Stalin's death in 1953 (there were general amnesties of prisoners in 1955 and 1956 as part of the de-Stalinization process), and many surely died while imprisoned. This was a fate unique to Soviet POWs, and one that compounded their already harrowing experiences of German captivity.

Sources:

V. N. Zemskov, "K voprosu o repatriatsii sovetskikh grazhdan 1944-1951 gody," Istoriya SSSR 4 (1990) [the main Russian source on repatriation of Soviet citizens]

G. F. Krivosheev, Rossiya i SSSR v voynakh XX veka: Poteri vooruzhennykh sil: statisticheskoe issledovanie (OLMA, 2001) [the main Russian source for statistical information about Soviet forces during WWII]

Christian Streit, Keine Kameraden: Die Wehrmacht und die sowjetischen Kriegsgefangenen (Dietz, 1997) [the seminal German work on Soviet POWs]

Reinhard Otto, Rolf Keller, and Jens Nagel, "Sowjetische Kriegsgefangene in deutschem Gewahrsam, 1941-1945: Zahlen und Dimensionen," Vierteljahrshefte für Zeitgeschichte 56, no. 4 (2008) [a more recent and more digestible general source on POWs, but only limited discussion of repatriation]

Unfortunately, there isn't much material in English on Soviet POWs in general or the issue of repatriation in particular, so I can't really make any recommendations for further reading unless you read German and/or Russian.