How did the peace treaties both follow and violate the principle of self-determination?
Some ethnic groups got self determination. These were mainly the ones who had been on the side of the Allies. For example, the Slavs got self determination in the new country of Yugoslavia.
But others did not get this. For example, the Germans did not. Many ethnic Germans ended up being ruled by other ethnic groups, as with the Germans in the Sudetenland of Czechoslovakia.
Also, non-whites did not really get self determination. For example, Vietnam wanted to be free from France and this was not granted.
I guess you could say that you got self determination if you were white and were not on the losing side of the war.