[Etaoin Shrdlu]

Or there's a magic site that deals curses that nobody's found (but Arco shouldn't have unseen sites due to easy access to Acashic Record). There'd be no surprise at such a site- there's at least one that deals Horror Marks to anything in the province- but you should've found it. As far as that goes, it ought to be a Very Rare site, so if this is happening in specific provinces more often than others, route your armies away from those provinces.
There is a spell that dumps curses on a remote province though- it's Baleful Star (in unmodded game ALT 5, S3, 7 pearls). Have you been using this perchance?
Even worse is Doom (ALT 7, S4, 100F+1 pearl)- the battlefield version. Curses the entire enemy army.
If this is all "vengeful witch" events there may be something odd with your RNG landing on that event too often to be by chance. And from my experience (not much admittedly) that event rarely curses more than about 5-10 soldiers.
Not really an answer, but maybe helping to frame the question correctly...