@Riot shop undo bug?

This isn't the first time i have posted about this but since i never got any clarification i am posting again. Why is it that when you buy the wrong item in fountain and hit undo your health drops back down to where it was when you first hit undo? if thats not a bug then, ok cool. But if it is i would like some clarification as to why this happens. Im tired of almost dying in my fountain to globals when i undo an item and get taken down low again.