I finished was in the top at the end of the round, but wasn't promoted to the next Stage!
Last Updated: 787d
One Championship Round lasts for 48 hours. Anything can happen at the end of the Round: other players won't sit idly and will constantly upgrade their machines to get as many medals as possible. It is possible that you were busted out of the top positions at the end of the round and didn't get promoted.
Better luck next time! You will definitely beat this Stage.