If I am on a rogue that is transformed into the Sandstone drake walking around Dalaran, I get dazed and stunned as if I am attempting to enter the rogue class hall while near the alliance inn. While mounted on the drake near the menagerie, I immediately get targeted, flagged for PVP and phased from the person who is the drake. It seems to maybe be picking up PVP phasing from the sewers below. This keeps happening several times recently on various characters and seems to be picking up phasing from below ground while you are mounted on the drake.
Edit: New weird developments with the sandstone drake phasing underground: I was mounted on someone as the drake who used the teleport pad from the upper to lower area of Oribos. Upon entering the bank area and being dismounted from the drake, I was ejected into the Maw as if I had jumped off from the upper level. Very bizarre!