Hello, I do not know if this is a known bug or not, but a quick search didn't bring anything
I used wring neck on a Mallard drake from a full crate and the animal simply disappeared. Both my crate and the inventory were full, with no space for the animal to fit in. I checked and it is not on the ground. See the attached pic for reference.
hnh.jpg
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"The psychological trials of dwellers in the last times will be equal to the physical trials of the martyrs. In order to face these trials we must be living in a different world."
Reproduced by myself - mallard dropped on the floor but was invisible - was able to collect it though by pressing 'q' in Amber (auto pickup). Maybe there is no in-game model for dead mallard of some sex?
Maybe it's client related then. I use Pasta client, that is based on Amber anyway. Unfortunately I could not reproduce due to not having another living mallard
just for the note: to reproduce it u should have no room for dead mallard. Maybe in my case it was just dropped beyond CB (the OP was doing it from crate as well) I was wrecking neck from, but my char was able to get from there...