Assuming that no work benches were removed the barn structure would have to be inside a work bench radius to construct. If a gray dwarf spawned outside the workbench radius it would have broken at least one wall to kill the boar. I suspect the boar didn't eat and died of starvation because the taming process hadn't started.

I use a covered barn for tame creatures (except for Lox. Those get an earthen walled pen that I access by earthen stairs).

Barn with isolation pen (for when i finally find a 2 star boar on this island).
https://steamcommunity.com/sharedfiles/filedetails/?id=2853688746
Inside the barn with the tame animals.
https://steamcommunity.com/sharedfiles/filedetails/?id=2853688768

The covered barn protects the animals from thrown rocks and flying creatures at least until I can handle the attackers.