the durability bug happens when you hit a dead body ?
As I understand it, it happens all the time, hitting anything really. Hitting walls and especially dead bodies gives more drastic results, but the bug is there the whole time because of the doubled framerate (60 fps vs 30 fps), that's how I've always understood it anyway.
I don't really know how the damage system works in DS2, the very code I mean, but it looks something like the damage is calculated per frame, so 1 frame of the weapon hitting a living enemy, dying ragdoll enemy, dead body or a wall receives a set amount of calculated durability reduction, depending on various factors and modifiers for the weapon and the character stats. The devs have fine tuned these calculations to work as intended at 30 fps. The PC version works at 60 fps thus the number of frames a weapon travels through an object is doubled. This doesn't necessarily have to mean that the durability reduction is exactly doubled, but it's most definitely increased drastically.
They added the durability reduction when hitting ragdolls and dead bodies to penalize button mashing, forcing you to carefully execute every hit. As I understand it, the same thing happens on consoles (never played it on them) but the penalty isn't as drastic.
It's a similar issue (though inverted) to versus fighting games like SF4. Every move has a set number of frames and it's crucial that you perfectly time your moves to execute combos, at 60 fps. If your rig isn't good enough, the game will have frame drops and make executing combos much harder or impossible (I might be wrong on all of this).
In any case, it seems to me that From Software made DS2 with these frame calculations in mind at 30 fps, without any planning on making the game work at higher fps. And as to why they haven't just increased durability on all weapons (or reduced penalty for that matter) I don't know. Maybe there's all kinds of stuff happening in the code during the calculations that might screw up other things, or it's just too problematic to have two sets of balancing stats in their production pipeline, I don't really know.