Shop undo bug

I don't know if this is a bug or if its supposed to be like that, but the other day i was playing a game and after a near death experience i recalled and bought the wrong item after almost healing to full health but when i undid the item i bought i went back down to the health i was at when i first got back to base and died due to a jinx rocket hitting me after i undid. so i went to a custom game to try it out built a lot of health and sure enough after being low and recalling and undoing a bought item my health went down to where it was when i first got there. so if this is a bug, riot... plz fix