Appears to be a bug with changing classes after you change out your creatures.
I change my creatures out at the menagerie, getting my reaver group instead of my cabalist group. I go to the wardrobe and change my class specialization to reaver, and when I come out of the wardrobe, I have my cabalist creatures attached again.
Edit: even weirder, it doesn’t actually switch out for the group I just had. It specifically switches back to my reaver setup, which is currently Menagerie group 2.