This havent been confirmed to be a bug or a feature, but all evidence leads to it being a bug. Otherwise, why isnt it mentioned as a drawback of the fire form in the skill description? Its probably got something to do with the animations, like the other major bug with skill casting that was found by BMAN! (YEAH, THE MAN). For those unfamiliar with this bug, its explained in another thread by bman but basically it had to do with the turn animation, it only does an inital check for the point to turn to, and if target have moved before turning is complete, it never did a re-check so the DG was frozen in place. This TB fire mode only bug is probably something similar, although not as major as that other bug.
Lets hope its fixed in patch 1.1!