Archangel's Staff upgrades before Manamune

I built Manamune then Archangel's staff on Pulsfire Ezreal in the first half of a Nexus Siege game. When you mouse over each item they display the same number of stacks. The Archangel's Staff upgraded first even though the Manamune built the majority of the stacks. I was able to recreate this in a solo Nexus Siege custom game. When I did so I noticed that basic attacks against towers or minions grant a Manamune stack. I didn't test to see if the skin used actually matters.

Steps to Recreate:

1. Start a Nexus Siege game
2. Pick Ezreal and use the Pulsefire skin
3. Start with a tear
4. Build a Manamune
5. Check current stacks
6. Basic attack a minion or tower
7. Observe that stacks have increased without casting a spell
8. Build an Archangel's Staff
9. Observe that both items display the same number of stacks
10. Observe that when 1000 stacks are reached the Archangel's Staff upgrades to Seraph's Embrace
11. Observe that the Manamune stacks have reset to 0 (Which they should)

The Archangel's staff also upgrades first when it is the first item completed.