Sumo Digital has already acknowledged the problem above and say they're looking into it.

I've mentioned this on NeoGAF, but we're looking into the Wii U Boost Challenge and Ninja Tag issue (and any other non-race game mode). To cut a long story short there was a problem with the update which went live on Sunday - and we're working to get it sorted ASAP.

Thanks to AutoSaver for the heads up!