I’ve collected more than 12200 coconuts 
for the event however the rewards stopped there. It doesn’t let me go further to collect 15000 coconuts for the bonus shaved ice seeding.
A question about this has already been posted in the Japanese page.
The official response states: “There are seven possible themes, but if you collect 15,000 coconuts, you’ll receive any color of shaved ice decoration. If you don’t receive one, please contact support.”