I am not totally sure if that is a bug, but I cannot see any reason why this is intended so I'll put that one here.

During a level 77 expedition I found the Zeitgeist vendor and she had an unique in store with required level 40 and 5 stars quality (Picture01). In comparison to that you can see my legendary staff, which I got during level 60+ expeditions. The unique is totally noncompetitive to any item that is dropping during expeditions of that level.
At this point I ran into that problem for the third time and because of that it is totally not satisfying to encounter Zeitgeist during the expeditions. So please make her items on stock (especially the uniques) scale according to the level of expedition where she appears. Otherwise would make her totally useless for the character-progression.

Picture01: https://imgur.com/MEuT8Wt