i killed the boss 15 times on heroic and it didnt drop the ax
And? If something doesn’t have a 100% chance to happen, and it doesn’t happen, that’s not a bug. That’s just the luck of the draw - good or bad.
Note that charts fans have put together which claim a 100% chance don’t mean it’s actually going to be a 100% chance. Without access to server-side data these are strictly guesses.