BUG : Mountain Drake Buff Double Damage

I just got out of a Co-op vs AI match just to get my win of the day where I was on Riot Girl Tristana, just chilling in the morning after Thanksgiving.

I was pushing in top lane to get the game to end, you know the usual LoL stuff. Then I noticed something. When I was auto-attacking the Red side top Inhibitor Tower I was dealing 18 true damage per hit (nothing new there). I had Rapidfire Cannon equipped because why not, it's only bots. When it went off however is when I noticed the true damage from Mountain Drake doubled from 18 to 36. We got Mountain Drake toward the end of the game so I wasn't able to notice it sooner and I was killing the last tower in the game so I couldn't test it on another tower. I also didn't notice it sooner on that tower because Rapdifire Cannon's effect went off to kill a minion just before I started attacking the tower. I did watch the numbers though jump toward the end of it though from 18 18 18 36 18 18 18 (towerdeath).

The reason I know this is a bug is because NOTHING should be able to alter the damage output numbers on True Damage. It has been stated numerous times, a number of them lately due to the Shyvana passive enhancing Smite's damage onto dragons bug that was on the PBE before getting fixed.

Match Details : 1st Dragon we took was Cloud Drake, 2nd was Mountain Drake Build - Rapidfire Cannon, Blade of the Ruin King, Refillable Potion, Lord Dominik's Regards, Berserker's Greaves, and Phage Level 14 with E maxed 1st, Q maxed 2nd, W 1st level'd level 4, ult leveled at 6 and 11.