I faced Justice Ironbriar again, and I managed to get a screenshot. Like da mayor, the increase in difficulty was applied to the summoned monster. In the screenshot you can see that despite the difficulty being arcane 14, it is clearly a bug, because the +3 in the difficulty is due to the scenario being heroic, and not his power.
No Pass-and-play
No Permadeath
Story mode (Never tried Quest Mode)
Seoni and Kyra were in two different locations (I don't remember which), Merisiel was at the mill
Turn order: Seoni, Kyra, Merisiel.
Scenario was The Cult Exposed, Heroic difficulty
I think it was the first explore of that turn
Noted this before anyone, including Merisiel, had played any cards to alter the check
No Horde
Screenshot: https://www.dropbox.com/s/ufclcecim1ekj16/2016-05-03%2000.48.04.png?dl=0