Dragon Age II 1.03 patch notes, balance section:
"Lieutenant-rank enemies can no longer lose more than 40% of their health from a single hit, and boss-rank enemies cannot lose more than 20%."
So, yes I'm sure.
And it's not difficulty problem, it's just about common sense. Greater part of rogue skills just do nothing with this damage cap. For example: Assassinate upgrades (Annihilate, Overkill) becomes useless because you will hit cap even without them.
There is another ridiculous thing about this change, dual-wield rogues calculates they abilities as 2 hits, so melee rogue can hit boss for 40% hp actually, which means that archers twice as worse than dual-wielders...