Today while running the cliffside fractal we had the hammer disappear from our hands twice, both times while a guardian was holding the hammer.
First time I was running up the ramp, had about 14 stacks of hammer corruption on me, and hit F2 to cleanse some conditions, which immediately caused the hammer to disappear.
Second time it was the other dragon hunter who picked up the hammer during the final fight, hammer disappeared from his hands while he was hitting a chanter.