Hey
I understand your point. However
It's not based on percentage rewards, it's constructed based on ranking and total damage.
a) The HP of the Nine Tails will be adjusted automatically according to the Total Damage as the battle progresses.
b) The rewards will be sent based on the damage a player manages to cause in proportion to the Nine Tails’ Total Damage.
The "A" is as important to understand as "B"
Also this isn't a bug.