Kassadin bug with presence of mind and ultimate, Can't cast 800 mana ult unless i have 800 mana

Not sure if this was just in my game, or if the bug is universal, but when i would proc presence of mind and have 4 ult stacks, i was unable to cast R unless I had over 800 mana, it wasn't making the ultimate cost mana, but i was unable to cast it for free unless I had enough mana in the first place? Really weird and maybe doesn't make sense but that was my issue. I have heard that this combo is overpowered on Kassadin, so it may make sense as a way to sort of balance it out, like to get it for free you at least have to have enough mana beforehand?