Janna Q bug

I finally captured a clear shot of this bug. It has happened to me many times and cost me a lot of deaths. When Janna tries to reactivate her Q immediately after casting with less than 90 mana (the cost of her Q) left after the first cast, it doesn't reactivate. I paused the replay after I casted my Q with more than enough time to reactivate but less than 90 mana left.