[GAMEPLAY] Undoing Item Purchases - Possible Bug?

Recently noticed that buying items from the shop, and then refunding them, returns your health and mana to it's pre-item-purchase amount, regardless of the amount of healing you have received at the fountain. Is this a bug, or something intentionally placed in the game? I have so far been able to recreate this situation 3 times since i noticed it, as Master Yi, Zed, and Ashe.