I haven't gotten the weekly Sterling Treasure from the Postmaster for a few weeks now. Possibly began around the same time I rolled a 2nd character. Can't find anything on this topic that isn't from back in April and already patched.
When Rise of Iron was released, Sterling Treasure mystery boxes were discontinued from being sent to players' postmaster each week. They are now only available from your first Prison of Elders level 41 completion each week.