Well, there were Trotskyite elements in the People's Liberation Army. If you had a situation were Japan was basically left to its machinations in China, and inevitably bled dry by the event - an uber-Vietnam of sorts, and the PLA liberates the nation, depending on who is in charge they might be even more mistrustful of Moscow and also more radicalised - pushing for the revolution into Tibet, Korea, maybe push to seize Macau if Portugal is still into its introverted pseudo-fascist state. From there these hard-leftists might look on Mongolia and see not only a part of Greater China, but also a country trapped by the wrong kind of socialism - cue invasion.
However such a China would probably be beaten into a bloody pulp by the USSR, unless it took the anti-Trotskyite move of holding its tongue for a decade or two to build itself up.