I cannot say with absolute certainty that it is a general “Haste” problem, but using “Fiery Breath” with Sarkham, then summoning a dragon with haste will not trigger the Fiery Breath ability for that dragon.
My current setup was with a Flameshadow Conjuring in play, I activated the ability then summoned an Avaricious Dragon. The dragon attacked with haste correctly but did not deal x4 dmg first
I don’t have Sarkhan, but I’m assuming that when you trigger his ability it only applies to dragons currently on the board and not subsequent ones that turn, even with haste.
Well it is obviously what happens here, but the wording of the ability does not mention such limitation
When your Dragons attack this turn, they deal damage equal to their Power times 4 to the opponent.
There is an event phase before the attack, I think it would be simple for the devs to put the triggering at that moment instead.