Is this a bug or am I the unluckiest hunter ever

It is a bug, one that's been described before.  The way to stop the animals from respawning after a kill is to park your army on it (use occupy instead of attack).  A lot of players like having this type of square near them because it offers them a ready supply of hides and animal parts.  The rest of us are just jealous.

It's a spawn point. I wouldn't call that a bug. There is one just south of my desert city.

Regardless, you should always occupy with your army after making a kill, so that there is no doubt who owns the hides and parts, and also so your harvesters don't get killed by NPCs or other players.

I've noticed spots like this; in my notes I describe them as "animal hot-spots"; but the "spawn point" hypothesis is highly credible.
It's marginally more efficient to send one army to attack (it'll get some loot off the kill - I'm always amused by the amount of gold wolves carry) with another close on its heels to occupy (so that your harvesters can harvest in safety), rather than setting the attacking army to occupy (it won't get loot - which is kinda dumb, but that's how the game works). One reason for this is simply that the right troops to occupy a square effectively aren't always the right ones to attack it; and the right equipment is always different (the weapons and armour with good defensive bonuses aren't the ones with good attacking bonuses; and, personally, I only give equipment to the defensive team, mostly because it's tedious to have to tidy up after the inevitable losses from the attacking team).