Library of fate (scrolls) 125-130, there is a mob with a tooltip that says "Cannot take deadly strike"
If you have helm enchant path of flames that adds deadly strike as fire pierce this gets weird behaviour you getting negative pierce instead. Re-equiping the helmet resets conversion and solves the issue until you meet next such mob.
Need to un-equip the helm > kill the mob > equip the helm.
I believe this might be the only place to cause this bug (due to the only place of such mob?)