I don't know the hell did they do or is bugged with the rate, but for all the people I know it has been very unstable. A friend of mine spend 20 pulls and has got 2 30% and 1 50%, meanwhile another do the same but not even a single voucher. But I still see voucher pop up a lot in the free pull. The rate (from my perspective not assuming pls dont ban me mud) is around 6%/voucher.
Anyway, if you wanna spend on the event I suggest you to wait until the 2 last day of the event. I have a strong feeling that kage ninja (or new rare) is gonna pop up in the last day of the event since there hasn't been any new rare for this week until now.