For me has been the opposite of you, after I reached lvl 70+ I've seen more Uniques and a few legendary items dropping. Zeitgeist (Unique Vendor) I've found her plenty of times during my campaign playthrough, plenty of times during my expeditions. I think rates are working just fine it's just that you have had very badluck and I've had very good luck.

Super very unlucky here to!

i will say the drop rating is way to high!!! as it is unique and shouldn't be dropping too often
but your just unlucky try clear hole maps as ther is a bug soo Zeitgeist dont show up on mini map and some time his hard to find
but im sure the unique will soon start dropping soo many that you have to vendor them cuse you dont have room for them any wher like rest
off all the players :P