Multiples times i've found some M9A1 bayonets and keep it in my inventory non attached on a M4, and always noticed that his statut will go to 'Ruined' after some hours of play without getting shot or anything (I don't even use it to open cans).
On the other hand, if get a pristine (or worn, or damaged etc...) M9A1 bayonet and attach it on M4 his statut will remain Pristine, no problem.
Get an M4 M9A1 bayonet, keep it up non attached in your inventory (backpack, vest...) Note his statut (Pristine/Worn/...) and play some hours.
Check it back later, tadaaaaa, Statut : Ruined.
My beautiful M9A1 bayonet *Cry*
I've had this with all kinds of actions, after a few hours of in my backpack (sometimes even when not getting hit by a zombie/player) some items change quality. I've had 1 canteen go from ruined to worn once too, that wasn't that bad.
Only happen to my M9A1 bayonets, got no problem with all my other objects/items in inventory.
Note that my backpack and equipments are mostly Pristine or Worn, I never take objects under theses statut
@Problematic
I've heard somewhere if you got a badly damaged/Ruined Backpack (or vest/holster) your items in it will start to degrade. Maybe is that your cause.
I have the same quality restriction as you do so I actually think it was atleast worn - but as it happened a few characters ago, can never be too sure.
I've had it happen on multiple occasions though, and the quality going up is weird in any case.
I personally don't think the quality system (as the rest of alpha) is not all that stable yet.