My guess on the armour decaying, is that the armour + plate decay is being subtracted from the plates first, rather than the entire armour as is usual. Your still receiving the same protection, and the armour isn't decaying so I can only assume that your plates are decaying with vigi and nemi decay added on. Which i think is pretty sensible, the plates are on the exterior of the armour and should take damage before the armour itself. If this a bug, then bad
MA. If this is by design, then it could be a boost to realism.