After I played Yogg Saron, Master of Fate, one of the spell casted was “Candle Breath” - Rogue spell. At this moment I have in my hand Dragon minion (Malygos). Nonetheless all 3 cards drawn did not have cost reduced. I think you should fix it. If it is OK, please explain why.
Hey there Tomers!
The card Candle Breath reduces its own cost by (3) when you are holding a Dragon, and not the cost of cards drawn. So basically Candle Breath costs 3 and not 6 when cast with a Dragon in hand - and that effect is not visible when the spell comes from Yogg Saron or other similar effects, as it is cast automatically without paying its cost. I hope that helps! 
And for the future - with English-language questions make sure to visit our EN Hearthstone Technical Support Forums: