Archangels + Manamune - A Bug or Intended?

If you build tear into Archangels and then buy another tear and eventually upgrade it to Manamune then the +8 mana from spellcasts persists on the Manamune. If instead you complete your Seraphs and buy a Manamune outright it will not retain the +8 from spellcasts and function as the tooltip reads.