Stalin had a policy that all Soviet personnel taken by Axis forces, whether p.o.w, forced labour, volunteer labour or Axis forces were to be repatriated to the USSR whether they wanted to go or not. As Allied POW camps were over run by Red army forces the 'liberated' prisoners were removed eastward to places of 'safety'. There were then placed obstacles in their path of repatriation so that they were virtually held hostage, your people for ours kind of thing. Even as groups allied personnels security was not safe. Individuals picked up by soviet forces fared even less well with an army quick to shoot its own troops never mind axis pow. So a certain number would have disappeared off the radar one way or another. Bearing in mind that soviet citizens returned by the western powers, even if being in axis hands were no fault of their own, were sentenced to 25 years Siberian time. The soviet system being a large un wieldy bureaucratic monolith was slow to move and even more reluctant to admit its mistakes so if an allied serviceman ended up in the gulag his chances of re appearing to spill the beans in the new cold war era were remote. Stalin when questioned about the missing 20000 Polish officers taken prisoner by Russia following its invasion of Poland in 1939, some who surfaced at Kaytan, said "ah we made a mistake there." The subject was dropped by the U.K and U.S governments if not by the Free Polish Government in exile in London. Much good that did them. When returning to Poland for 'free' elections under Stalins Guarantee of safe conduct they to were wisked away to Moscow and murdered.
Having said that there are records of allied personnel being reasonably well treated by Russian forces. Stamford Tuck ,in fly for your life, gives a description of being with soviet forces following his liberation. He eventually got home. The pianist didn't.