As far as the Our Missing POW's of WWII website's claims, besides general incoherency and and inability to make the case is concerned, may I refer you to an earlier Does the ≈400,000 figure for US soldiers killed in WW2 include the ≈80,000 MIA? where the issue of accounting for the missing and missing in action is addressed. Ultimately there were just 6,058 US Army, including the Army Air Force, missing in action, worldwide, who were unaccounted for in some other category and were thus declared under the requirements of Public Law 490 77th Congress.
Some interesting reading here http://www.fdrlibrary.marist.edu/_resources/images/mr/mr0157.pdf on the subject. While some strenuous complaints about on the scene repatriation obstruction are made, it appears that the majority eventually made it to Odessa where after some Soviet hospitality they were eventually returned to US control. If you open this, scroll to the bottom and read the documents going up, they are in reverse date order, newest on top.
Did the Soviets keep any WW2 US POW's? One might suppose they could, but to what point? And being liberated from a POW camp meant that someone, another prisoner, would know that someone was missing and would say so. Not to mention military folks are notorious records keepers and list makers; the senior POWs in the various liberated camps, not to mention the Germans, knew exactly who was in each camp and their status.
One might note that of the crews of eight USN PV-1 type patrol bombers that fetched on Kamchatka during operations against the Japanese, seven crews were repatriated and one entire crew lost in a crash (wreckage discovered in 1962 and examined by US personnel in 2000 see https://pacificwrecks.com/aircraft/pv-1/34641.html). Also repatriated were three B-29 crews and one B-25 crew who landed at Vladivostok. While the USN PV pilots and the B-25 pilots probably weren't all that overwhelmingly interesting from an intelligence point of view, it would seem that there should have been at least a modicum of interest, intelligence attraction, for the pilots of the most advanced heavy bomber in the world, or perhaps some USN or USAAF airborne radar operator types, but, no, such interest and they all came home . . . meanwhile the Soviets were busily reverse-engineering the B-29.
The US Army final accounting of casualties notes
Captured & interned total = 124,079
Captured & interned, returned to military control = 111,426
Captured & interned, killed in action = 3,102
Captured & interned, died of wounds and injuries = 453
Captured & interned, died of other causes = 9,098
From “U.S. Prisoners of War and Civilian American Citizens Captured and Interned by Japan in World War II” found here https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/u/us-prisoners-war-civilian-american-citizens-captured.html#count
“ . . . Charles A. Stenger, formerly with the Veterans Administration (VA), developed a set of figures revised annually since 1976 for POWs and an estimate for current numbers of surviving POWs for the Department of Veterans Affairs Advisory Committee on Former Prisoners of War. According to Dr. Stenger, these figures were compiled in cooperation with the Department of Defense (DOD), the National Academy of Sciences, and the National Archives. They are recognized and used by the Department of Veterans Affairs (VA) and other government agencies. Dr. Stenger lists 27,465 POWs in the Pacific, of whom 11,107 died while in detention. “
11,107 deaths of US military and naval prisoners dying while POWs of the Japanese. The US Army reports, above, 9,098 of its personnel captured or interned as dying while in detention.
The survival rate of prisoners held by the Germans (from which, obviously, those liberated by the Soviets would be drawn) can also be found at the above site: “. . . Dr. Stenger's figures list 93,941 U.S. military personnel captured and interned by Germany, of whom 1,121 died . . .” Since only about 15 POWs held by Germany were US Navy personnel, were we to conclude that the 1,121 deaths were US Army, we can subtract that number from the US Army reported deaths of 9,098 and arrive at 7,977. Compare that to the 11,107 total US military and naval POWs deaths while under Japanese control.
This site http://lcweb2.loc.gov/frd/tfr/ has a plethora of documents, mostly relating post WW2 which address sighting of American service personnel from the Korean War Era and Cold War incidents. There are also long lists of civilians being held, pretty much starting with the post WW2 era. A search of the sub-site U.S.-Russia Joint Commission Documents Database (http://lcweb2.loc.gov/frd/tfrquery.html) using the search phrase “US” yields, for example 26 documents. Admittedly only breezing through the list (which, again, is primarily post WW2 oriented), one of them however, TRF39, is a long listing of downed USAAF aircraft and the disposition of their crew members. Although there are apparently numerous translation errors, a repetitive phrasing of note in the document TFR39 (http://lcweb2.loc.gov/frd/tfrussia/tfrhtml/tfr039.html) is “. . . for transfer to the America Military Mission.” For example, there is “By the order of the Commander of the Third Ukrainian Front, Marshall of the Soviet Union Comrade TOLVUKHINA, I am sending you 76 American and English airmen, who were shot down in the rear of the 3rd Ukrainian Front, FOR TRANSFER TO THE AMERICAN MILITARY MISSION.” A list follows which if you bother to sort through it – spread sheets are handy for that - you will find 75 USAAF B-24 pilots and crew of 8 B-24s and one USAAF P-38 pilot (there are no RAF/Commonwealth types on the list, looks like the Soviets were confused as to the origins of the various missions).
In this same document one finds included those killed or hospitalized as a result of aircraft losses. There was apparently an organized effort to return personnel to US control/authority. This is not really my field, (WW2 US Naval Aviation) so I don’t spend a lot of time on it. Attention is invited.
The difficulty, then, is coming up with a verifiable number, much less the names, of liberated US POWs or recovered US personnel retained by the Soviets at the end of the war. A quick and dirty rule of thumb is that the absence of evidence of XYZ is not evidence of XYZ. So, could the Soviets have retained American service personnel liberated from German POW camps? Maybe, perhaps, but it would be nice to see real evidence rather than that Prisoner A heard from Prisoner B that there were Americans in the Tayshet Gulag.
Certainly to go to the trouble to retain former POWs and deny, deny ad nauseum, said POW ought to have some value besides knowing how to drive a jeep.
Is it true the Soviets deliberately withheld liberated US POWs WW2? Probably not, perhaps more of a boogieman tale from the Cold War.