In game 23742103, between just me and Lord Rattington, I played a Skulk which elicited Plague as its Hex. LR gained a Curse, presumably to its hand as is supposed to happen. I then played a Mountebank (having played both a Plaza and a Blessed Village earlier that turn) and it gained a Copper, even though I'm pretty sure it could have discarded the Curse to prevent this.
As you can see in the log, the bot did not get a Curse due to Plague (there is no mentioning of it in the log, and card gains are always mentioned). The reason can be confirmed by reloading the game: The Curse pile was already empty at that point.
Oh, doof. :-[ Thank you for responding. :)