This is not so much a solution, just a note to those like me who won their first championship and popped the 'Brand New Champion' but not 'International Series' (even though I won every race on the tracks listed).
I went back into 'Championship' in the single player menu, and reloaded the championship I had just completed. After selecting 'Finish Championship' the game credits began to play (again).
After sitting through five minutes of credits the achievement popped (the credits were still rolling). Not sure if mine glitched because I skipped the credits the first time or if it is due to some other reason but there you go.