How it's possible that if an Elk has very visible big antlers, then when you kill it, there aren't?

When you kill it it comes with the hide.

If you look at the elk, it will have Hide i believe with antlers rapped with it.

Either that or its a bug?

Its strapped to the hide, yeah

From time to time it drops as the ingredient "large/small antlers". It seems random and something to do with a loot table, is not going to appear the 100% of the time.

Maybe you hitted the antlers and they shattered, so that's why you cannot loot them.

For me is just a random sh*t....