Haha, never seen anything like that. I'm sure it's related to devilutionx. Actually, Scavenger's Carapace has an effect which reduces its AC by 30, so since the breast plate's ac range is 20-24, this results in -10 - -6... provided that negative values are allowed because otherwise 23-30=248. :-D
Afaik this was actually a bug brought over from Hellfire, though it's possible that it's easier to trigger. But yeah like Qndel said it's already fixed.