How many times have you completed / won this race and yet it still shows that you haven't won the race? And NO, Nachi run is not bugged.

Let me keep this short and simple, look at the bottom of the screen:

Select Switch to normal event with whatever your key is set to, and do the race again.
And just like that, the race now shows that you’ve won the race.

It’s a pretty bad design since the first popup leads you to believe that you’re doing a normal event, but you’re actually doing a broken / bug season event.
End of guide.
