I think that’s a case of incredible luck again.
Saw a couple of cursed treasures while doing some runs…
Oh, by the way, are you certain you were doing Heroic bounties?
Even had to do some digging…
A new option has been added to the Mystery nodes on Heroic Bounty maps. Whenever you land on a Mystery Treasure node, you’ll get additional rewards if you successfully complete the Bounty! But beware, much of that treasure is Cursed, and will hinder your characters throughout the rest of your run. However, every once in a while your Mystery Treasure will turn out to be straight Bonus Rewards: all the additional prizes at the end of your Bounty without the added Curse! So… are you feeling lucky?
https://hearthstone.wiki.gg/wiki/Patch_23.4.0.139963#Mercenaries_Updates
Apparently, that was supposed to be the condition for it to appear.
PS Oh, forgot to add: I doubt they’ve changed anything at this point.
Maybe the system detects what you’d want and denies it, that’d be more plausible. 