Tear of the Goddess/Manamune/Archangel's Staff Bug

I recently noticed a bug with and its two improvements ( , ). I started out on Aram with one (playing as Kog'Maw). I later got a second . I immediately noticed that both of them read as having the same amount of leveled mana (~100 or so) even though I had gotten them at different times. I built one into , but I noticed that the second would level when the leveled. I later built a Soon, the leveled into , but in doing so reset the mana count on to zero (which began leveling normally, but was quite annoying in that late game).

Thank you for your time.