first man survived going into space
While the USSR was fairly secretive and during its existence, a successful hush-up of a major screw-up was at least possible, we would have learnt about it since 1991.
Some documents are sealed but many were declassified since then.
Apparently the USSR told the story as: he kept sending back scientific data up to the last moment.
PS. They definitely hid some of their failures, but I don't think it would have been possible with loss of life involved.
James Oberg was uncovering info about phantom or missing cosmonauts in the early 70s, including photos of the original class of Cosmonauts before & after some were airbrushed out. At least 5 men were removed from the photos and they have all been traced (Grigoriy Nelyubov being the most high profile as a member of the "Sochi Six") despite Soviet space officials going to significant lengths to excise these men from their history.
Since the Fall of the Soviet Union we've had revelation after revelation about embarrassing episodes in Soviet history (including previously covered up issues with Vostok 1 during its flight), yet we've discovered nothing about further missing cosmonauts. Of course there could be much better covered up episodes, I'd just expect we'd know a hell of a lot more about them by now.
(As an aside, this info is from Oberg's 1988 book "Uncovering Soviet Disasters" which has _fascinating_ little passages about how much more open the Soviet Union has been "recently" with regards to publicising disasters under Glasnost and how that will continue into the future. So interesting to see as someone born shortly after the fall of the Soviet Union.)