That's how it's supposed to be but I usually get another from the completion rewards

Yep, it's only 1 guaranteed per account. Been that way since it was added in

Typically I'll get 2-3 by the time I'm done with all the bounties on all 3. But not always. But it's 3 chances per character with the hunting wolves bounties. Then seems like a chance turning in each of the other three bounties per character. Usually can net2-3 but not if I really need one. Not that that really happens. Have all the queens weapons so don't need anything out of that chest. Can still get exotics from PoE without opening the big chest. So I fail to see much reason in opening it. Anymore.

I've been getting one per alt per week (total of 3). Not sure why this is not happening for others though.

Yes, but apparently that's how it works... Ridiculous, if you ask me. Some people have been lucky with RNGesus when opening ether chests and PoE chests, me..not so much. The fact that the rewards are garbage soften the blow...