That may not even be Lenin’s real cadaver.
I suspect the actual corpse had become desiccated and simply fell apart some forty years ago, like a top heavy Sand Castle.
What is on display is some inert, non organic material that is less likely to attract vermin.
Wax, polyester or acrylic. There is likely a vault jammed full of Lenin replacement Corpses. Once in a while, the Cleaning Lady’s string mop will wrap around the wheels of the display case and tip it all over.
It is said that only the skull in the mausoleum actually belonged to Lenin.
Following his death they removed his brain to study it and determine the source of his communistic genius.