Kassadin's R Bug

I was playiyng a Legend of the Poro King match with Kassadin and I used his R - Rift Walk, but is consumed more mana than needed.

The conditions which this happened are described below: -- I had it stacked at maximum before using the skill again; -- The timer of the stacks wore off moments before I used the skill; -- The mana cost, due to the stacks, was at 800, when the normal cost is 50.

When I used the skill, the mana cost, which should be 50, was 800 and it generated the first stack of the skill.

The match was at the Brazilian server: http://matchhistory.br.leagueoflegends.com/pt/#match-details/BR1/680135695/1529813?tab=overview