I'm sorry i don't agree damage doesn't stack.

STS can confirm this fact too.

As players we might have discovered differently for xp elixirs in general those all stack indeed. But I don't believe Damage, Armor or Speed stacks at all and goes by the highest multiplier, you are more than welcome to use all you damage elixirs at the same and time , just know only the highest one will come into effect.

I did not reach the leaderboards or even attempt to for Gauntlet events knowing bear class is not as effective and that is the only class i can play now a days.
However, if you found a bug that allows the damage stacking of the damage elixirs that is wonderful and beneficial to you and your groups of friends who did top 10 the gauntlet event. It is great you discovered that bug and secret, and reaped the rewards.

However in standard pve maps from my experience and testing Damage only goes by the highest multiplier which of course the only vanity sets that damage does stack are the three i mentioned in the post above. So I don't see it as a bug at all. And it was not how the game design was intended.