Just downloaded a new update for this game. I believe that's the second one in the past month or so. Do we know what these updates are fixing?
https://en-americas-support.nintendo.com/app/answers/detail/a_id/29365/
Fixed an issue where after clearing stage 3-3, the route to stage 3-4 would not open.
Looks like it was an old Wii U patch - wonder what took so long to bring it over.