As for testing with vanilla, do a fresh install to another folder.
Unfortunately, once installed, Steam Launcher won't give the option to install to another folder, so I just deleted all mod files and ran integrity check to force the Launcher to re-download the vanilla files. Hopefully that would be enough for a vanilla gameplay.
I tried to reproduce but couldn't make trigger the bug on my end. Also, checking the code it's very hard to find out a reason for something like this to happen.
Hmm. McKwack, feel free to send over the save file for testing and I'll try again. As a wild guess, it could be something related to the area where the action takes place, or in what sequence the preceeding actions were performed.
Agreed that the bug is tricky to reproduce,
but somehow I managed to consistently reproduce it by creating new characters, start with the "Not All Who Wanders are Lost" starting scenario to make sure that you would always have a starting
guinea pig animal that you can butcher.
Also, save-scum does
not re-trigger the bug. i.e. I copied a save file before I attempted to trigger the bug. Once the bug was triggered and I loaded the pre-bug save file, the bug won't trigger anymore..

At any rate the save files are too large to be attached to this post, so I uploaded them to my Google Drive for easier share. Here are the links to the saved files:
- Lutung: the character with whom I first found out about the bug. Have moved around quite a lot because I thought I simply misplaced the missing items.
- BAC-Tester: test character using BAC mods.
- Kaumo-Tester: Vanilla (or as Vanilla as I can get) test character.
Again, it's highly likely you won't be able to re-trigger the bug on any of these characters, although I was hoping that there'd be some hints of what happened in the log files, if any.
Please let me know if you can't download any of these save files.
Thanks Sami.