Running 3.71 unmodded on Windows 10 via Steam.
I just started a new Finn and on my 3rd day got an elk. I butchered it and saw the elk fat in my inventory. I then went to break some nearby ice, and from looking over the message log, that's when the fat disappeared. I can see it in the list of items I dropped - but then when I picked them up I cannot see the fat because there were too many items, but where it says "+ 7 items" I see that is 1 fewer than I had dropped. My fat was gone!
I noticed a day later when I went to tan the skin, so recent save game available.
EDIT: Any suggestions as to how to mod in a kilo of elk fat would be very gratefully received!