Muramana Bonus AD from maximum mana bug

When i had 0 deaths, and I bought muramana on ezreal and started stacking it, it gave me 20 ad to start with, but as i stacked it, it stayed as the 20 extra damage for 2% of my total mana, even tho my mana reached above 2 k, once i died however it went to the correct ad of 45 before death proof: http://i.imgur.com/18Bh0Hw.jpg