I just started my infiltration of Arkemyr's manor only to discover that the robes don't work as a disguise. I've gotten the clue about using them from Fassima, and the corresponding journal entry. I've snuck into his sleeping quarters, stolen the robes from the closet and put them on, just as always, but both the iron constructs and the imps act as though I'm not wearing any disguise at all. The dialogue entry where you use the "cider and cheese" passphrase to convince them you're Arkemyr never shows up in the text, and constructs go hostile immediately upon detecting me.

There's another thread on here about the same issue. The developers are aware and are looking at a save file 

confirmed, wearing robes not trigger dialog with imps

 

 

confirmed, wearing robes not trigger dialog with imps

 

 

 

Have the same issue with Arkemyr's Robe, though in my journal there are no entries except for this:

"I've made it inside Arkemyr's manor. Now I need to locate his vault." I think this is because I did enter his manor once through the bathhouse when I was just new in Neketaka and doing some exploring. 

I also got the key from Fassina, as well as her tip about using Arkemyrs robes, but the game doesn't register this it seems. 

Aaah, that explains a lot.  I also thought something was wrong with robes this time around, but since I managed to sneak all the way from undergroubd to Arkemyr's office to get vault combination, and up to pickpocket the imp, I did not investigate further.