Please report to helpshit - Contact Us — Conan Exiles Help Center - so the devs can see it sooner. (But please also continue reporting here as well so that issues can be added to the player-maintained bug lists.)
The greataxe may only drop from the overseer. I’ve gotten a couple from him.
Got the dual axes once from the chest at the blade master. Not sure if chests can have all the weapons or not. Seems to be a very small chance of a weapon regardless.