The Veteran Ard Banser is ending up trying to climb up the cliff. It is stuck to this task, and usually cannot climb back up on its own, leaving it open to all attacks until it either makes it up the cliff from external action(part breaks, rage mode) or dies. Not sure how it ends up there, it should spawn near the cliff with the lvl 15 phongruf mobs. My guess is it perhaps wanders off the cliff or runs off while chasing players, but once it gets off the cliff, the bug occurs as it tries to get back.
possible workaround: Make the lower part of the cliff also be part of its "fighting area" so that it isn't making its way up the cliff?