I think i am in the same situation as the OP. Here is what i did: Made a new char and played the tutorial. Got the "World Warrens Three" achievement in the Tutorial (lots of new kobold models in the Mines of Gloomindeep), which rewards the kobold banestrike. Went into The Warrens and tried to use my banestrike on the old/classic kobold models there. Got the error message: "You havn't mastered this race ... Between step 2 and 3 the kobold models in the warrens were changed with the may 2017 update. As the OP, i am suspecting a connection between the model change and the dysfunctional banestrike.
Noticed trhe same thing, but after the patch of july 19th, the kobold banestrike works again in the warrens. Did i miss something in the patch notes?
