Are the items aboveground? Aboveground organic materials will slowly degrade in freezing or scorching climates.
Sorry, very late reply.
Yes, they are above ground. And yes, I'm aware that refuse can decay... but across many embarks and many years, this was the only time I'd ever seen a bone furniture and gut rope decay. I've let gut ropes sit in the tanning workshop for years and they don't disappear, likewise non-refuse items made from bone (furniture, blocks, weapons, other parts). And refuse items, when they decay, just up and vanish - they don't go through the damage stages I saw here.
I think what might've actually been happening is that the specific tile was somehow hot, hot enough to slowly (quickly) destroy the items... but only on that tile. It was in a scorching biome but, as I mentioned, not only is nothing else breaking down like that but the workshop surviving just fine and the benches getting completed without problem when I built the workshop a few tiles distant from where it used to be. I don't even know if temperature can work like, but it is the most plausible theory I have.
In any case, I long ago abandoned that site since I'm having trouble actually building a fort - I keep getting distracted by things that I'll end up having to move when I get around to working on what I actually want to do.
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New question: do warlocks have any buildings that open bags of loot from butchered kobolds?
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Also also, IMO warlocks should be able to specify making hourglasses from glass (in the glass furnace), not only from rock boulders at the craftsman. Unless it's a balance thing?