After the failed reaction I could see a message 'Milith soldier cancels pickup equipment : type mismatch'. Queuing another stygian bronze blade whip works as intended, providing a weapon.
Gathering the bug reports, it seems that the forge reaction is made of old code so I would not surprised if old bugs persist inside.
I cannot get it to work at all. Starting from that same save, I toggled on the civilian alert (to stop the bowsuccubi from going back up on the western wall and being shot at), and queued up a second stygian bronze blade-whip in the same magma workshop where the first one was due to be created. I waited, and the first non-bar job was completed, and the second was begun immediately, also with no bar. I waited for that to finish, then queued up a third stygian bronze blade-whip in the second magma forge, and that one ALSO was begun without a bar. While that was going on I queued up a stygian bronze duel sword job in the first magma forge, and that one was started WITH a bar. When it finished I had a +stygian bronze duel sword+ (weight 1, value 1170).
I tried queueing up an iron blade-whip, and that job was also started without a bar.
Every weapon that I have tried to create has worked except blade-whips. They fail every time.
Maybe this is something that only works in Windows, and not in Linux. But it's incredibly strange that it is so consistent and so isolated to this one type of weapon. A broader vanilla DF bug with weaponsmithing would have been noticed long ago, one would think.
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Looking at the raws, [NAME:blade-whip:blade-whips] appears to be missing a [MATERIAL_SIZE:x] tag, which the other weapons have. Maybe the Windows version fills in some default value here, which the Linux version does not?