Iksar said that in the new passive treasure offering system there is no way to have 100% chance for a certain treasure, but you will always get an offering that works with your deck. I dont think he lied but there is a problem with how Scepter of summoning is coded. It does something with "5 or more cost minions" and OP had 5 or more cost minions. So the game thinks that Embercaster and other 5-drops are affected by the Scepter so its fine to offer it.
I also had this kind of bad rng in duels, they really need to fix their spaghetti code for Scepter. Also instead of having a high chance for a treasure that works really well with your deck they should just eliminate the ones that do nothing.
Treasure choosing should be about, well, choosing. If 2 of the 3 options do nothing there is no choice at at all.