The answer is in the Soviet files.... some of which are now open and available to the public. But it’d probably be a massive amt. of research to dig through those files.
You are probably right. I have always thought this was an interesting mystery. I did at some point read that the post WWII KGB guy in Warsaw was Leonid Brezhnev which might explain why he was not released if he lived into the 1960s or 1970s.
Wikipedia says about Brezhnev, “At the end of the war in Europe Brezhnev was chief political commissar of the 4th Ukrainian Front which entered Prague after the German surrender.”