Russians have a history of miscalculation.
“The North saw this as the South drifting apart, both as a separate country and into capitalism. They didn’t want that. And now that the Americans had left, Korea was defenseless. Kim Il Sung asked Stalin for support for an invasion of the south. Stalin was hesitant, but thought: The US did nothing to prevent the fall of China to Communist hands. They will probably do nothing here, either?”
https://unchartedterritories.tomaspueyo.com/p/why-did-korea-split