No idea what caused this, but my prisoner's Rags and Chasity Belt ended up not having any durability when I started a dungeon. Not it being set to zero, the meter didn't even show up and her accessories took the durability damage instead. It returned to normal when I cleared the dungeon and checked it in the guild, and I don't think the game would have let enemies equip items on her because it still had the yellow border in the adventurer overview, meaning the game likely still saw it as unbroken, but I didn't chance it.
Update: just started another run with the same character, the Rags and Belt still don't have their durability bars...
🎶On a scale of 1 to 10, my friend, you're fuc-🎶