undoing a purchase in the shop sets health and mana back to what they were when you bought it

in a bot game [beginner, I think; I was teaching a friend] playing as jhin, i recalled, bought zeal when I was at about 300 health, decided to get BF sword instead, so I undo, and notice that the health and mana bars go back to what they were before I bought it. I quickly test it again by undoing the purchase of my bf sword too, and it happens again.

I'm pretty sure that's not supposed to happen.