[Gameplay] Bug when you undo an item in fountain

When I make a purchase in shop , I have , for example, 300hp . I wait till I regen Some hp and I decided to undo my purchase. I return to 300hp , it's like I undo my regen and it is disturbing in late game. I was wondering if it's something that Riot was aware of and just couldn't fix it or if they have just not paid attention to it.