You are not the only one, same happend to me. I saw in another thread that there was a way to get the item through a console command. Although, this mister forgot the command...
I saved after the matter, so I can't go back, but this man mentioned that, to fix this, you can beat the hideout just before you talk to Arzagos. Once you talk to him, youclear the hideout again andyou should get it.
Sadly this is no option for me. I hope it is for you though!