I’ve actually noticed this for quite some time (probably since Custom Settings were added), but when you choose Custom Settings and modify things, it appears to impact the ability of Deer to re-spawn. As the hunter shoots and kills the deer, eventually the spawn point disappears and you have no more deer to hunt. As you move the hunter through the map to a new area, the same thing happens. The deer are hunted to “extinction” and then you have to move on again. They never come back.
Wolves do re-spawn.
I don’t know if this affects boar.
Note that this happens even if I leave the “Deer Multiplier” to Normal. I don’t recall if moving the slider has any impact…honestly, I usually leave it at Normal.
I play “Arid Highlands” and use Custom settings to remove resources (clay, iron, gold, coal) to increase the difficulty. I don’t know if this bug happens with the other terrain types.