i got the max score and the game ended abruptly, but it wouldn't let me press Z on the end scrren, so i guess it didn't save it? :(
EDIT: anything past overachiever triggers only once you hit the "lift curse" button from the shop (tested it with the 9999x medal).
however, once you hit the max combo, the game immediately ends mid-game, giving you a special gameover screen (without going through the shop).
the button to go to the menu from there doesn't seem to work, but i'm guessing that's irrelevant because the score gets updated when you lift the curse and not when you reach the menu.
TL;DR because you can't lift the curse when you reach the max combo (the game immediately ends, and even if you could, you'd need to spend combo to lift the curse and go below max), the medal for max combo is effectively broken, which explains why no one got it since