I don't understand how buffer states worked all too well, and I'll use Mongolia as an example. In the cold war era, it was supposed to be a buffer state between USSR and Red China. But those two powers already had a shared border to the east, so wouldn't a buffer state be negligible, if they bordered each other?
Mongolia was not established for the specific purpose of being a buffer state and, in any case, didn't serve as one. The Soviets had forces there, which pissed off the Chinese to no end.
Mongolia achieved autonomy after the 1911 Chinese Revolution. There was a lot of jockeying for influence between Imperial Russia and the Chinese Republic, but after the Russian Revolution the Chinese had the upper hand in shaping Mongolia. They considered Mongolia to be a part of their State as the successors to the Qing dynasty, but the Mongolians wanted independence.
At the tail end of the Russian Civil War a White Russian military formation under Baron Ungern-Sternberg (a brutally fascinating character) crossed over and set up the Boghd Khan in Urga (now Ulaanbaatar). Soon after, the Red Army invaded to crush the Whites.
There was Bolshevik agitation in Mongolia since the Revolution, but the formation of the Mongolian People's Republic was set off by the Red Army invading to crush the Whites. So it wasn't set up as a "Buffer State", but as a conquest of the Red Army as part of the Civil War and a spreading of the Revolution.