Bug Report - Archmage

I have a level 19 Archmage gem, which, with the quality bonus, provides 8% additional damage. I corrupted it for +1 level and added the Lightning Mastery support gem. Now it's at level 21, and it shows 9% additional damage. However, the DPS of my active skill (Spark) has not increased, and in the additional information (the arrow icon) next to the Archmage gem, only the mana cost has changed. When Archmage went from 7% to 8% additional damage, both numbers increased, but when it went from 8% to 9%, they did not.

Same issue