Viktor Hextech Core Shop Bug

Today in an ARAM, I bought the first tier of Hextech core as Viktor. Nothing odd about things so far, I upgraded one of my spells and was off on my merry way. After dying, I found myself with enough gold to purchase a Perfect Hextech Core outright and skip the tier in between. After I bought the Perfect Core outright, the game gave me one chance to upgrade a spell, and after that nothing. So I had to run around with 2 spells upgraded and a Perfect Core. If this is an intentional design choice, and you HAVE to buy each tier of the item separately, there is nothing in the tooltip that would indicate this. That also seems counter-intuitive and even a bit stupid, if you ask me. The only rational conclusion I can draw is this is a bug. If not, I would love to know to avoid this occurrence in the future. Thank you.